So you have 1900 out of 2000 favor/xp left for donation. And you have 1000 meat. So normally, you would have to donate once, seeing if favor would be wasted, then calculate the exact amount and donate it. But if there was an option that let you donate based on amount of favor left, I think it would be much easier.
"You have 1900 out of 2000 favor/xp left. The exact amount can be achieved by donating xx meat. Would you like to donate this amount?"