Percentage is only worthwhile under two conditions.
First, you see the rate con and know exactly what the load was paid. If not, then you’re getting a percentage of who knows what.
Second, you can control what loads you take and you have the knowledge of what lanes pay. Otherwise you’re at the mercy of your dispatcher and can get stuck on a string of crap paying loads.
Like as already said, you will probably wind up averaging the same as milage. Or maybe worse, maybe better. It adds another variable to the crap shoot that is performances based pay.