Pay Stub Issue When Retired

My experience and others I spoke with whom were self employed-
Provided most recent traditional paystubs(one ......... back several years), recent signed contract with one of my current clients showing my current rates, and my expectations for salary.
I believe that the number they came back with was based on their assessment of market rates.

I doubt that they offer less than what you can prove that you made in the past if the position was relevant.