I do not know how much the recipient should keep, but I still think it would be helpful for him to know that it's independent of his current salary. For one thing, it's awfully suspicious for the amount he should keep to be that close to the amount he earns.
Things like motivation-breaking also have to be integrated, but it's not enough to justify keeping 99% of his money. At that point, almost nothing is lost of he loses motivation, so the benefits of greatly increasing donation would seem to easily outweigh the risks.
By the way, you were implying that giving money to poor Africans is optimal altruism.
This is clearly false.
By doing so, you merely make up for other people's policy failures and create perverse incentives and moral hazards for the causes of poverty. In addition, other people are more likely than you to give money to the poor rather than to abstract causes, which means that you have an intellectual comparative advantage in focussing on those other causes.
I think all of the following are better candidates for hed.utils. than poverty relief:
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.