@Wily
That's the sort of mental calculation I'm thinking of though, if it makes any sense.
I understand your considerations. I have made similar calculations myself. You can justify planning out a wargame for a seven-figure income and an "optimized" annual givings, running utilitarian calculus on whether to give a nearby homeless guy $50 or donate $50 to a water charity in Indonesia, but quite frankly, people who do this kind of mental math rarely do it right and they end up either with decision paralysis leading to them just selecting whatever is easiest or they end up talking themselves into some weird scheme which ends up being less effective than they hoped for.
This is because this optimization approach is, more often than not, an analytical approach to the problem, rather than an insight problem. It represents a cognitive bias towards persistence and fails to utilize the brain to its fullest of creative problem solving. Analytical problem solving can be highly accurate but it is often less accurate than insight problem solving based on domain expertise and time spent doing something.
Understanding oneself and one's surrounding are essential to understanding one's Being in the world and what one can do, based off of that knowledge.
May I suggest that you should grow your awareness of the present state of your community? Not to make any negative assumptions, of course, but I find that many people asking about "what can I do?"- in the analytical fashion that you are- are often not fully aware of what's going on outside their front door. Learning more about the people and things around you will be invaluable in providing hints and information as to what you might be able to do, now and in the future, to help things, near and far.
Anything you care about on a "big picture" scale is happening right now in your community. Often times, the conditions of your communities issues and the solutions thereto are the same in issues and solutions very far away and of a much bigger scale. Master the ability to learn about and address local issues and convert that domain knowledge into insight solutions to novel and larger problems.
If you're running this sort of calculation on whether to spend some small amount of your week doing something small but useful or talking to neighbors for a few minutes when you see them, then I question how much you actually want to do it vs how much you're trying to talk yourself into wanting to do it. This is the analytical mindset at work, high serotonin and high estrogen.
The idea is to maximize that ratio to the best of one's intelligence, because doing anything other than that would be negligence or 'bad faith' (you COULDVE done more... think 'I held the door open for someone so my good deed for the day is done')
I understand but I will stop you here and say that placing your engagement with charitable acts under the criteria of "could I have done more?" is going to be something which will end up torturing you because the answer is always yes. Analytical solutions needs ten minutes to arrive to something that insight quickly determines as "yes." You will have to learn to live with that if you want to do anything. To do more is why we do things, at all. Life exists in order to do more things. Humans have hundreds of unique differentiated cells in order to do more things. To do more things is fundamental to the universe.
A truly "charitable" existence is not wherein you've carefully designated certain hours of the week to being in a specific place or a meticulous accounting for charitable giving. A charitable existence is something wherein you will be influencing people and things around you, for the better, nearly 100% of the day by seeking and living a high metabolism life. Think about charity as a life process, not a singular or recurrent action or thought pattern. It is something which, ideally, is embedded into your existence at fundamental levels.
You could take a college class in algebraic topology but insight will grant you a vision of how you can use that knowledge to help others. You could use your insight to write a short and thoughtful ebook to boost your resume and secure your job position so you can reliably donate an amount of money while setting up for future actions. Regardless of what you are doing, if you are of good metabolism then you will not be complacent in your life, you will want to do more and the question of "could I have done more?" will not be a looming shadow but something else entirely.
Do you like animals?