Yeh manufacturers always quote engine dyno crank hp as it industry standard .
In reality you can not accurately calculate wheel-hp to crank-hp by percentages as each transmission design has different windage, drag, mass, gear cut design and so on making them differ massively . also you have measure points in a transmision which is steady state (constant fixed speed) and dynamic (accelerating), Both of those will incur different power loses.
also loss will not be linier to input hp, more input hp will create extra loss in certain areas such as increased friction from load .
A rolling road uses run down drag with fine tuned mathematics and known values for car models and while potentially offering good usable ballpark figures in ideal machine and operator circumstances still won't be spot on .
You will see massive differences on R tranny losses from varying RR as they all differ, even same RR model and software will differ pending on condition of machine and operator and climate if massively different .
Condition of transmission and parts used will make difference too, changing drive angle through 90deg in final drive and diff ring and pinion is biggest power loss so mechanical condition of these on a 25yr old tranny plays big roll as does aftermarket gear kits being used and types of lsd diff fitted. Oil grade and actual level also big player, lot of variables so would need do lot of tests and pre checks to build up realistic usable numbers.
Bob a good option on input on this as he got a decent spec and pretty new RR and handling large number of R's so using that data for loss of hp figures and wheel figures of standard car vs manufacturer hp figure could be useful in calculations and confirming RR mathematical crank-hp accuracy .
I always used around 22 to 25% as rough ballpark guess on loss for the R standard tranny, it is nothing more than rough guess though .