Depletion is a odd, thematically and mechanically.

Thematically, it doesn't matter at all if a planet is 99% depleted. But the moment it hits 100%, a sudden environmental collapse takes over and you suffer a huge penalty! (The only thing that makes sense here is that at least Cravers lose their FIDSI bonus for working undepleted planets.)

Mechanically, this means Cravers have a ton of micro to do - idealy you drive every planet to the brink of disaster, a bunch of slaves behind (and maybe one Craver) so that it never quite reaches the brink before the game ends. Ideally you never actually deplete a planet at all. 

The AI is hopeless at this game, but human players can sidestep the entire depletion mechanic if they move their Cravers around. 

Solution:

Scales the FIDSI penalty of depletion from 0% up to 100% of the penalty with the % depletion that has happened. 

That way you'll still want Cravers to gobble undepleted planets up (for their sweet, sweet bonus on undepleted planets), but there's no 'magic' point you'll want to stop your Cravers eating the planet. If you want more resources, you have to pay a price. How long before you decide to find fresh planets depends purely on how much you value short-term gain over long term depletion (as it should be).