C. S. Lewis’ dictum is useful and challenging: if we aren’t giving so much that we have to go without something we want that we would otherwise be able to afford, then we aren’t giving enough.
That being said, there isn’t any point giving so much that you’re crippling yourself from being able to continue giving, either. (Unless you’re making a choice to effectively sacrifice yourself for someone else’s sake as an emergency martyrdom effort; but that’s for unusual super-special-case situations.)
Personally, in today’s world I tithe from the paycheck, since I don’t consider the other money to have been ‘paid by me’ to the government; companies take that stuff out from the outset and we never see it. On the other hand, I tithe from the gross of income that I would be taxed on later; and I tithe on any money I get back from the government as tax refund (or whatever) since I hadn’t tithed on that already. Once it’s in my power to actually give something from it, I do.
I’m also in a position where I can give between 25% and 33% of my income to charity each month, which I hope to increase the percentages of over time; coincidentally, it’s also roughly what devout Jews were expected to tithe when they had the ability to do so: the 10% is a starting point.
But, not everyone can do that. And the main point is to try to help other people. Give prayerfully, and don’t kill yourself doing it; you should be okay.