As long as you are measuring the thrust in calm wind and on a level surface, you will get the same static readings on a 12 pound plane as you would a 20 pound plane. Just remember that static thrust is an entirely different animal than dynamic thrust, so don't be suprised if your, say, 12 pound plane doesn't have very good pull out with, say, 20 pounds of static thrust. Once the plane starts moving, that thrust is now called dynamic thrust. The prop has unloaded and the resistance to movement has been substantially reduced (you are no longer holding the plane back). This is why a plane will hover at half throttle but still not have great pull-out even if you have a 2:1 static thrust-to-weight ratio.