Rest assured that a Webra 50 with the 12.25 X 3.75 APC will have plenty of excess thrust to fly a 5 lb plane.

Don't get too involved in trying to measure thrust on the ground. Try a few different props and see which ones fly the plane best.

Cheaper than a set of fish scales and it's a lot more fun to fly with different props than measure a plane tied to a post.

This is just my take on the matter....but if you really want to know, I have a Irvine 53 with a UT muffler, turns the 12.25X 3.75 at mid 14's. It has just over 10 1/2 lbs static thrust, measured with fish scales.

The program you mentioned is Thrust HP and it said I should be getting 14+ lbs. I was sceptical, so I measured it. I fish also, so I have scales. I find that the Thrust HP numbers X .7 are fairly close.

But I still went out and bought a couple different props and tried them.