The below may sound harsh but I hope it is understood from taking a more high level view. I laude your effort and altruism but to make it effective maybe another tithe is not the right way.
So I'd like to put the tithe somewhat into perspective. Originally the tithe was the income tax. My reading is that at all times the collected tithe (together with other taxes) was used partly for charity, largely for functioning of society (infrastructure, defense, education, ...) and partly for presentation/status as well as luxury of the rulers.
Basically the same with our modern taxes. And yes: There were different additional kinds of taxes at different times but until recently few income taxes.
Now I asume that philantrophy has existed at all times and some people have given more voluntarily. And social pressure has surely worked its way too - especially when there is no fixed amount but a (partly) publicly visible process. But I doubt people called additional donations 'tithe' too. So strictly the usage of tithe is semantically wrong and to me it looks a bit like justifying extra donations as historically normal which it isn't. Understandably so given the cause.
Our modern taxes are significantly higher than historical income taxes. Currently e.g. < 20% for normal incomes in U.S. and < 30% in Germany (maybe higher income taxes are part of the explanation why Germans donate less? In total they apparently contribute more).
So why do we have to add an individual charity component and 'manage' it outside and in addition to the existing structures (government, tax) for this purpose? To me it looks as if we have given up on government to deal with charity. Wouldn't changes to govenrment and taxation be a better service for society?
Actually there are kinds of charity that cause zero to net negative effects via the reduction of social systems. An example are food banks which take responsibility away from the social system thus allowing liberal policy to save in that area.
See e.g. Tafeln und der Abbau des Sozialstaates (pg 144).
I fear that adding charity just skews the existing systems and adds an additional element the effects of which nobody sufficiently understands. Each new system element in a complex system tends to bring more problems than it solves.
Our modern taxes are significantly higher than historical income taxes. Currently e.g. < 20% for normal incomes in U.S. and < 30% in Germany (maybe higher income taxes are part of the explanation why Germans donate less? In total they apparently contribute more).
That's a common explanation. Another is that the US has a culture of giving to churches; Germany for its part has a church tax, which won't show up as charitable giving either.
I made a comment on another site a week or two ago, and I just realized that the line of thought is one that LW would appreciate, so here's a somewhat expanded version.
There's a lot of discussion around here about how to best give to charities, and I'm all for this. Ensuring donations are used well is important, and organizations like GiveWell that figure out how to get the most bang for your buck are doing very good work. An old article on LW (that I found while searching to make sure I wasn't being redundant by posting this) makes the claim that the difference between a decent charity and an optimal one can be two orders of magnitude, and I believe that. But the problem with this is, effective altruism only helps if people are actually giving money.
People today don't tend to give very much to charity. They'll buy a chocolate bar for the school play or throw a few bucks in at work, but less than 2% of national income is donated even in the US, and the US is incredibly charitable by developed-world standards(the corresponding rate in Germany is about 0.1%, for example). And this isn't something that can be solved with math, because the general public doesn't speak math, it needs to be solved with social pressure.
The social pressure needs to be chosen well. Folks like Jeff Kaufman and Julia Wise giving a massive chunk of their income to charity are of course laudable, but 99%+ of people will regard the thought of doing so with disbelief and a bit of horror - it's simply not going to happen on a large scale, because people put themselves first, and don't think they could possibly part with so much of their income. We need to settle for a goal that is not only attainable by the majority of people, but that the majority of people know in their guts is something they could do if they wanted. Not everyone will follow through, but it should be set at a level that inspires guilt if they don't, not laughter.
Since we're trying to make it something people can live up to, it has to be proportional giving, not absolute - Bill Gates and Warren Buffett telling each other to donate everything over a billion is wonderful, but doesn't affect many other people. Conversely, telling people that everything over $50k should be donated will get the laugh reaction from ordinary-wealthy folks like doctors and accountants, who are the people we most want to tie into this system. Also, even if it was workable, it creates some terrible disincentives to working extra-hard, which is a bad way to structure a system - we want to maximize donations, not merely ask people to suffer for its own sake.
Also, the rule needs to be memorable - we can't give out The Income Tax Act 2: Electric Boogaloo as our charitable donation manual, because people won't read it, won't remember it, and certainly won't pressure anyone else into following it. Ideally it should be extremely simple. And it'd be an added bonus if the amount chosen didn't seem arbitrary, if there was already a pre-existing belief that the number is generally appropriate for what part of your income should be given away.
There's only one system that meets all these criteria - the tithe. Give away 10% of your income to worthy causes(not generally religion, though the religious folk of the world can certainly do so), keep 90% for yourself. It's practical, it's simple, it's guilt-able, it scales to income, it preserves incentives to work hard and thereby increase the total base of donations, and it's got a millennia-long tradition(which means both that it's proven to work and that people will believe it's a reasonable thing to expect).
Encouraging people to give more than that, or to give better than the default, are both worthwhile, but just like saving for retirement, the first thing to do is put enough money in, and only *then* worry about marginal changes in effectiveness. After all, putting Germany on the tithe rule is just as much of an improvement to charitable effectiveness as going from a decent charity to an excellent one, and it scales in a completely different way, so they can be worked on in parallel.
This is a rule that I try to follow myself, and sometimes encourage others to do while I'm wearing my financial-advisor hat. (And speaking with that hat: If you're a person who will actually follow through on this, avoid chipping in a few dollars here and there when people ask, and save up for bigger donations. That way you get tax receipts, which lower your effective cost of donation, as well as letting you pick better charities).