Here's my numbers as example:
FWHP = 608
RWHP = 474 so I divide 474 by 608 = .779 (call it .78).
With this reasoning, I have a 22% loss from transmission, driveshaft, rear end, wheel inertia, water pump, alternator, power steering pump, air cleaner, ambient air temp. (chassis dynos are usually in air conditioned rooms), mufflers, pipes, etc.
I can now back into my post-NOS FWHP by dividing the new RWHP number by .78 to arrive at FWHP from RWHP:
575/.78 = 737 FWHP (I usually roudf up to 740
.
So what this all means is that you need to have a FWHP AND RWHP to know for sure.
Brian