Drive train losses are a function of the load in the transmission, the normal force between the gear teeth increase at higher loads, and the friction loads at that interface are roughly proportional. Their are fixed losses as well which dont scale with speed.
Applying a gross percentage as an estimate of drive train losses is a rule of thumb at best, and for entertainment purposes at worst. There are 100 million related discussions on the internet. Nobody uses that number in the engineering context. The good thing for car dynos is that torque/power at the wheels is what matters for car performance, so the crank hp is just a curiosity anyways. If you really need to know it, better grab your wrenches and book an engine dyno
