Not sure how to simplify this, but sort of a natural log kinda thing:
=((1+(1/1-FC))^(1-FC))-1
This variant is simpler and gives a really hard drop:
=(1-FC)^e
Both give you a decay from 1 to 0 as FC goes to 100%.
@mav – I’ve added yours into the sheet too:
= 2/(4^FC)
Edit: play with it on google sheets! – add more 
Yeah, I’m bored …
