Load Calculation

When I needed a new roof, it took me over a year
to find a rep who knew roofing.  One rep told me
the job would be a roof over and would need 24
squares.  I knew the roof already had two layer
and would be a tear-off - plus the roof was only
24 squares.  Needless to say I am leery of home
improvement reps.

But now every one with whom I have spoken about
a new AC unit has a Master License (I checked).
But not one of them has done a load calculation.
And the lowest bid for a 2.5 ton, Seer-15 is
$5,400.

Where can I find the math for a load calculation?

Dick