direct is 1:1 rpm/torque , for brake HP you need to adjust the rpm based on the gear ratio. Simple calculation If I measure 2132 rpm and have a 5.7:1 ratio it's 2132x5.7 =12,152 RPM at the crank .Direct drive doesn't need any calculations for gearing, just the correction factor.
Then factor in rear gear ratio. A direct engine to dyno it a 1:1 ratio. That's how we did all the engines at the machine shop