15% is the standard rule of thumb for parasitic loss. It would be a little bit more for an auto. But a dyno is a dyno, there are many factors that go into producing a final RWHP. There is weather, temperature, mechanically variation on the dyno, mechanical tolerances in the particular engine.
But 15% is a decent guesstimate for what crank HP will translate to at the rear.
2011 Mustang GT M6 3.31s
Matt HONEYCUTT Tune, Lethal Off-Road X, GT500 axlebacks, J&M LCAs, UPR UCA, Whiteline LCA relocation brackets, Eibach Pro-Kit Springs, Koni STR.T shocks/struts, GT500 strut mounts, SR Strut Tower Brace
Best 1/4 E/T (05/18/14 MIR - no tune, X-pipe, or UCA): 12.920 @ 109.40 mph (2.059 60ft)