The only thing I see wrong with your test is that the model needs to be on a paved surface. You need to totally eliminate the rolling resistance from the setup. Uneven ground and grass will lower your thrust measurements. How much? Who knows.

As for prop selection and static thrust, the prop that pulls the hardest in a thrust test like this may still not be the prop you want to fly with. It will, however, give the best punch out of a hover, or otherwise the most thrust at very low airspeed. As you bring forward speed into the equation, the prop's efficiency changes. If you generally fly at a moderate speed, the prop that pulls the hardest will be maxed out, while one with more pitch and less diameter will be in it's element. I have proven this to myself with my Sukhoi. The 18-6 prop gives the most static thrust, but the airplane flies much better with a 16-10. Both props turn 9000 rpm, and the 18-6 definitely pulls harder on the ground.

Static thrust tests are good for figuring the thrust to weight ratio of your model, but not much more than that.