As others have pointed out, thrust calculations as described provide ballpark figures at best.
Another important point is that static thrust is just about useless as an indicator of flight performance, unless hovering is your main interest in life.
As an example one of my old OS46SF engines provides about 4.5 lbs of static thrust using an 11x7 APC prop. The same engine with an APC 12.25x3.75 propeller provides an extra pound of static thrust, i.e., 5.5 lbs. In hovering flight the 12.25" propeller is the best choice, but for traditional aerobatics it is just about useless compared to the 11x7 propeller.

Propeller selection is still very much a trial-and-error business.

/Red B.