Formula if we speak about a full % Arc lvl 80 donation: i.e. 280 x 190% = 532 as you use the calculator.
280 is, in this example, the chest value of your position 190% is what an Arc 80 can donate without actually losing fp’s.
This could also be 280 x 180% = 504 or whatever is common in your guild or among your friends.
If you like to grab a position and you just want to make interest on a neighbor you will need to fill 50% of the 100% left and if that is an interesting deal you can figure via the calculate as above.
Important note is what
@Gouls mentioned: make sure you can’t be passed by current donators.
Is the interst worth your time to wait for leveling is also a thing to keep in mind.
Also you can calculate what is already donated and what remains and if possible split that and make your donation nobody can pass you in such case unless there are other donators obviously.