RMy part would be the same for every cake, i would take the order, discuss it with the contractor, collect payment, bake, and fill most likely, then my part would be done. I know what it costs me, and an average of how long it takes to do my part, and I would still have my own cakes to bake and fill. The only variable is what the customer is paying, so that part the contractor earns would be the variable. I'd really prefer not to over complicate things, and would pay myself the exact same thing for doing it per cake order. The rest would go to the contractor. Simple.
This would be an extremely generous compensation package for your contractor, since you would be essentially reversing the roles of entrepreneur and employee by collecting your own costs and a small fixed profit for yourself with all the additional benefits accruing to the contractor.
If your primary goal is to have as simple a compensation structure as possible and you are not concerned about the opportunity cost of lost profits then your formula will work for you.