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.