Fly it. There is a difference between static thrust and dynamic thrust. Putting a fish scale on the tail and taking measurements isn't going to tell you how the plane is going to pull out of a hover or how fast the vertical speed (upline and downline) is going to be. To give you another idea, a turbine engine might put out 30 pounds of thrust, but it won't accelerate as quickly as a propped engine that also has 30 pounds of thrust. This is because the turbine is moving less air but at a higher speed to get it's thrust, while a propped engine is moving more air at a slower speed.
Like I said, buy a few props that your engine is capable of swinging and just fly the plane and see which one suites you better. I have gone through 7 different props on my 35%er and am still looking for the perfect prop, although I am currently only using one of those 7 props because, through actual flight-testing, I have found it to be the best of them for my flying.