RWHP to Crank HP reduction

[69znc]

I have been researching the percentage reduction for a manual transmission car between the flywheel/crank engine dyno HP vs the rwhp on a chassis dyno. I was only looking for empirically data and not "the rule of thumb". I never trust rules of thumb without some science! Knowing there lots of variables, starting with the engine dyno usually done in optimal conditions to every aspect of hp reduction points on the car. After many hours of searching the best I have seen with people trying to address this question is an 18.5% reduction. Most empirical comparisons were actually in the low 20%'s. Way more than the "rules of thumb". Any thoughts on this topic? What is your experience?