One factor you didn’t mention is that significantly harder working people might need to spend more income to maintain their productivity. E.g. traders working 80-hour weeks might spend more on food because they don’t have time to cook, more on clothes because their job requires signaling, more on airplane tickets because they have short-notice time-sensitive tasks requiring travel, etc. But there’s still no reason this should work out to anything like proportional giving, so your point stands.
If you only add this effect while keeping the rest of the reasoning the same, the amount you should give is your discretionary income minus a constant, rather than your total income minus a constant.
A compromise that I find appealing and might implement for myself is giving a fixed percentage over a fixed amount, with that fixed percentage being relatively high (well above ten percent). You could also have multiple “donation brackets” with an increased marginal donation rate as your income increases.
One factor you didn’t mention is that significantly harder working people might need to spend more income to maintain their productivity. E.g. traders working 80-hour weeks might spend more on food because they don’t have time to cook, more on clothes because their job requires signaling, more on airplane tickets because they have short-notice time-sensitive tasks requiring travel, etc. But there’s still no reason this should work out to anything like proportional giving, so your point stands.
If you only add this effect while keeping the rest of the reasoning the same, the amount you should give is your discretionary income minus a constant, rather than your total income minus a constant.
A compromise that I find appealing and might implement for myself is giving a fixed percentage over a fixed amount, with that fixed percentage being relatively high (well above ten percent). You could also have multiple “donation brackets” with an increased marginal donation rate as your income increases.