Hey worm,
It depends on whether it residual base for reoccuring contract/agreement or Salary + (sales - quota).
If its residual on a reoccuring contract, say annually, then 2 good methods to keep employees is 50% (of profit) or 35% up front of annual (with a clause of deduction for unexpected lack of return) + 15 % monthly.
If it is salary with a quota draw. Decide what the return should be for a certain salary, then give a bonus residual for sales beyond that point for not less than 20%.
Hope that helps. I own a sales company so if you need any other specific advice, feel free to e-mail me.