I'm in New York right now and don't have it to hand but there is a formula you can use based on the max rpms and the prop size and pitch that will get you very close to actual horsepower. Someone on here should know it. What you are suggesting measures static thrust and is done by almost everyone I know. You just run a nylon strap around the tail of your plane, hook it to a Berkeley Digital Fish Scale, and run the throttle up. That's where all of these static thrust numbers are coming from. They provide an initial measurement but the actual thrust in flight is different from static. The static thrust number comes in when you are going to hover. That's what let's you know if you can pull out of a hover, or even do one for that matter.