Rear wheel HP?

Is the conversion from flywheel FWHP to RWHP a standard 18% loss for a automatic car? I was just on the Moparts calculator. Is it still the formula. I never had the engine dynoe'd but im going off the vehicles weight and ET and MPH its ran. thanks

I usually see 22-28% with a auto depending on converter. 18 is usaully a stick car.

Thanks Jake68.