I've found that if you run the car down the quarter mile if its making MPH, its making horsepower. No two ways about it.
Driver error, traction, shift points, gearing all effect the ET but will only effect the MPH a bit and always to the milder side. Never makes it MPH higher.
3 or 4 good passes tell the story. All you need is an accurate car weight w/driver. If you make an engine tuning change and the car responds with more MPH, you're makin more power.
I've found that with Desk top dyno if you enter the estimated real airflow consumption in CFM of the motor in question instead of carb cfm its much more accurate. Most people over estimate on the choices in the cylinder head choice menu.
eg Stock head stock ports and valves refers to the best stock unported SBC heads, (like a set of 461X or untouched 034 bowties with 2.02's) not any stock junk head. all others are "low perf" Once you know what each menu choice actually flows, its suprisingly accurate, expecialy if you can determine the actual valve "running duration" (cam doctor and a bit of math)
A "400hp motor" will consume around 550-575 CFM if real airflow on the dyno.