Interesting idea. I'll have to give it some thought. Of course, being able to specify the rake as a percentage is really just a minor convenience. You can always specify the rake as an actual amount instead of a percentage. So, in your example with fees of $50 and $10, the rake would be $6 since it was specified as 10%, but you really want it to be $5 because the $10 fee shouldn't be raked, so just specify the rake as $5 instead of 10%. A little inconvenient, sure, but wouldn't that work?