If you understand how to do the test, do it. Regardless of what the RPM is at stall, write down the RPM and the reading on the scale. Adjust the clutch to hold the engine (stall) at a different RPM (up or down), take another reading, writing them down. Keep doing this till you find the stall RPM that produces the highest reading on the scale. The highest reading will tell you at what RPM (stall) the engine is producing the most torque at the axle. If you can, monitor the air density. Higher air density will produce higher readings on the scale. It's no different, actually, from testing your engine on a water/hydraulic break dyno. Any kind of dyno, acceleration or break, will tell you at what RPM your engine produces the highest torque reading. I 1st learned how to do this from a guy named Larry Moral. He built a hang glider called the Easy Riser. He did this exact same test to find the maximum thrust of a powered unit he had just developed. This was back in the early 80s.