There is no magic formula, because there are just too many variables. SOme include geraring, condition of lubricants, mass of rotating parts, manual or automatci transmission and a few others.
Usually, you will see a 15% loss from crank hp to whp on a manual and 20% on an auto. This is just a rough estimate, but tends to be a rule of thumb.
2006 Mustang GT in Torch Red
C&L Intake, Evolution Performance Tune, Borla Axle Backs, Hurst Shifter, 4.10's, JBA Long Tubes, Prothane Engine Mounts, Steeda UDPs, FRPP Charge Motion Delete Plates
84 Bronco, 302 C6 on 32 10.50's