I don't think there is a formula for an exact number. If I'm wrong somebody will correct me. But, there are so many variables. What kind of trans do you have? Fluid weight and wear of internal parts will make a difference. Same is true for the rear end, u joints, wheel bearings and every moving part in the drive train.
I think a ballpark figure might be 20 to 25% loss?

Welcome to CHR by the way bul.