I do think the answer is along the lines of actually creating a two sided market. Let the leasor and the leasee determine an acceptable price...
Almost like a bid/ask of a stock market...but in this case, the bid/ask will be based off STEEM POWER per STEEM and which will determine a fair APR.
This way, you guys don't have to worry about setting up what is an acceptable return is...it's all on the users and you guys get your 10% cut for basically being the escrow.
Coding a market is really hard and lease offers don't really work because we have to make sure the lease actually goes through, so there are many edge cases that have to be handled. But in the long run it will probably evolve to that.