F350 -Typically I use 12% for a Mustang with an 8.8 and a T5 as I have been fortunate to have run an engine on the dyno then have it run on a chassis dyno when installed in the car. Jim Bell told me the he usually figures a transmission like a 4r100 will take 50 bhp at full tilt. A 9 inch Ford will take about 3% more than an 8.8 if all else like ratios are equal. A truck can lose 20% or more at full power.