There is no 'standard' loss. If you want to know what your engine puts out then pull your engine and put it on an engine dyno. You can get an approximate idea through a dynamometer with a run down, but that's as close as you'll get.
As an example, if you had a 15% loss on a 200 hp engine then you would measure 170 at the wheels. If you added an intake worth 15 hp at the engine you would measure 185 at the wheels-not 182.75. Conversely, that 185 measured power factored into that math problem suddenly means your engine is supposedly making 217.65 hp.
Skip the ricer-math and stick with the charts you actually get-not the ones you wished for.