Here's how to determine the commercial assignment fee for a property that you have under contract. 

.75 x (Proforma Value - Actual Value)

The proforma value is the value of the property after raising the rents and applying the other income activators. 

The actual value is determined by NOI (net operating income) divided by the Cap Rate for the area. 

.75 is used because you want to make a nice profit while leaving some profit on the table for the investor that you are wholesaling the deal to.