RE: 50cc = 29 lbs thrust
Ih82crash is right
The line from hook to hing to scale contact point should include a 90° angle, and have equal length. The angle between pull cord and the line from hook to hinge should be 90° as well.
That can be achieved by placing the scale table higher.
The lever should presens as small a surface to the prop wash. Now that wash reduces the reading, and may well cancel the high reading by the (possible) angle errors