Reading over the other comments, I think a lot of this is about finding the right schelling point.
This past summer, I put a bunch of reminders spaced out by a month or two into my google calendar that say "make effective altruist plan" - the idea being to make some sort of contract with myself before I graduate and get a regular income again, and sit down and think about what goes into that contract many different times before actually "signing" (which will probably be showing it to a trusted friend or two and asking them to help hold me to it with social pressure). I'm probably at an advantage timing-wise, since I'm able to think about it while having some idea what my finances will be like (I've lived on my own while working a long internship, so I have a rough idea about groceries and rent and things), and before much chance of lifestyle inflation and hedonic adaptation happening.
I expect most of my donations will happen once I've got my career figured out. Right now, I think that will mean having a collection of small business activities that I can live off of, and end up with a really large surplus if I do well which I could donate, but something totally different could happen if I find I'm not cut out for self-employment. The contract is mainly about setting something up so that I stay in the habit of donating large-ish amounts, so that I won't be as likely to feel uncomfortable and change my mind if I get rich.
10% seems to be the most common schelling point, so I started from there. But that didn't feel like it was leaving me enough surplus to save as business-starting money. The current draft of the contract says 5% donated at whatever time of year it makes financial sense to (I know a lot of people base donations on tax seasons), 5% saved in an account that's only to be used for investing in things that I expect to be worthwhile profit-wise (likely my own projects). Money I use out of that savings account would be recorded, and that amount would eventually be donated later. If I don't use the account, it gets donated.
If you're worried about unpredictable expenses (like medical bills), maybe the charity-or-specific-other-use savings account would make sense for you? Also, if you've already got a full budget, looking first at where you'd cut back to make room for charity might make more sense than abstract percentages.
I'd like to hear from people about a process they use to decide how much to give to charity. Personally, I have very high income, and while we donate significant money in absolute terms, in relative terms the amount is <1% of our post-tax income. It seems to me that it's too little, but I have no moral intuition as to what the right amount is.
I have a good intuition on how to allocate the money, so that's not a problem.
Background: I have a wife and two kids, one with significant health issues (i.e. medical bills - possibly for life), most money we spend goes to private school tuition x 2, the above mentioned medical bills, mortgage, and miscellaneous life expenses. And we max out retirement savings.
If you have some sort of quantitative system where you figure out how much to spend on charity, please share. If you just use vague feelings, and you think there can be no reasonable quantitative system, please tell me that as well.
Update: as suggested in the comments, I'll make it more explicit: please also share how you determine how much to give.