You're not.

Seeking to 'bring it back empty' is work for the driver (true tank balance, estimating miles needed to complete the journey) and wholly impractical for short duration/low mileage trips. It's a sucker's bet.

I have seen - in Europe - agencies offering refueling at market-based pump prices plus a flat service fee of maybe $6, for those who don't want to interrupt their travel to refuel the car. That seems like a civilized way of doing things - but cuts profit potential for rental agencies so I fear there's no chance it will catch on in the U.S.