Wouldn't a more direct equation be quota = FYE/TAM ?
If you're expecting your reps to close 20% of the leads they get, then set their quotas accordingly. Don't reduce their commission base - you're running a very real risk of dramatically under compensating your star performers, and overcompensating your dullards.
Quick example - Joe Slimeball only closes half the business he should, but he would W2 (according to your numbers) 67.5k. Sally Awesome manages to find a few extra customers and makes sure they are happy with your product (yeah - sales people should be doing that too, it's not all about the tech, there's a human side to FYE) so she closes 30% of her leads. Her take home (depending on accelerators) would be 82.5k-90k.
3x the performance, and a few $$ extra? That's great way to lose your best reps to other companies, and encourage the rest to idle along at minimum effort levels.