There is no magical number for drivetrain loss. For every car it is different, even for identical makes and models. There's too many variables to try and guess a %.

BTW what DemonSVT said is correct. There is another way that is maybe a little easier:

Wheel HP * (1 + drivetrain loss %) = Crank HP

Example:

150whp * (1.10) = 165bhp

Isn't math fun?

Dan Acura Integra GS-R w/mods Former owner of a 95 SE