I think you might have been on the "fixed dish" (non-motor)
Sadoun calculator page. Click on the site where it says "if you have a motor click here..."
When you add the motor, the dish calculation is based on a formula:
(example, for the
STAB motors) it can be either
P - (60 - latitude)
or
p - (45 - latitude)
I cant remember what the formula is for
SM3D12 right now, but for example if you use top forumuala you get this:
p - (60 - 40)
For columbus, P= 43.8
So 43.8 - (20) = 23.8 to set the dish elevation to.
Thats very close to the 24 you have in the manual PDF.