It's not just impact though. There are sites which try to research the most cost-effective causes.
But that feels mechanical and hurts the "feel good" effect. While you can probably feel good by donating something closer to home that has a 70:1 effect compared to a 101:1 effect, for example.
There's also some other selfish factors to optimize for like tax breaks and divine favor.
And sometimes it's more effective to work for a good job as compared to taking a high paying 'evil' job and then donating that money.
But yeah, I try to systematically do all of the above. As far as I'm concerned it doesn't matter how long I live or how much money I make, only how much benefit I contribute to the world in the end.