Drawing up a contract

I act as a digital marketing consultant for clients. I would like to create a contract that allows me to guarantee they achieve either a certain percentage increase in gross profit (perhaps minus marketing costs) or sales.

I have issues here:

1. I need to have it so that they can't fiddle the numbers.

2. I need the legal wording done for me, but in a template form so I can change the values (such as my fee, sales/profit target etc.)

If you can do only 1 or 2, that is fine, just specify it in your bid as I can hire multiple people. Obviously I'm looking to sort out the finer details to prevent numbers being fiddled before the contract is created.