The problem with back-calculating to get an engine number is the fact that drivetrains don't take a percentage, they take a fixed amount. While that is indeed a percentage, that percentage lessens as power increases. Basically, if you gained 10 hp from an intake (for example) your drivetrain would not consume 1.3 hp from it simply because it is making more power.
Another example is in changing fluids within the drivetrain. If you dyno'ed a certain number on OEM drivetrain fluids and then changed to better fluids and gained 3hp to the wheels, you didn't increase engine performance-you reduced your drivetrain loss. You didn't change a percentage that can be used in future calculation.
The only way to know engine output is to pull the engine and dyno it as a stand-alone unit. If you want to know your drivetrain loss you would then have to reinstall it and dyno again, subtracting the result from the first number. If you then installed an intake and gained 10 hp at the wheels, you can pretty much guarantee that if you pull the motor and dyno it again that you will see the same 10hp increase in the engine-alone dyno.
All things being equal, I could care less what my engine produces because I've never seen an engine win a race without the rest of the platform attached to it.