Everything I've seen is it is a constant proportion. If it was 20% stock it will be 20% when modded unless you change something about the drivetrain. A more effecient tranny or differential (lighter components) can reduce the percentage.
New Ride -- 2015 Z06 2LZ (stock) -- Journal
Old Ride -- 2012 Camaro 2LT/RS (647 RWHP & 726 RWTQ) -- Build Thread