Don't rely on formulas you are not sure of. Calculate or measure it on your own. The formula depends on the type of fixed roof you have. Cone, dome, umbrella, etc. Take your shape and calculate the projected area of a slice through the highest point (usually through the tank center). If this is a mystery to you, consult an engineer familiar with the term "vertical projected area".
Right offhand, I can't think where vertical projected area comes up in current API-650 calculations.
First equation, looks like pt is roof slope per foot. The 1/48 is a factor of 1/2 for OD to radius conversion, a factor of 1/2 for area of a triangle, and a factor of 12 for inch to foot conversion.
In the second equation, it looks like a factor of 1/4 is missing. The tan(4.76) = tangent of (4.76 degrees) = 1;12 pitch.
It should be triangle area = 1/2 x base x height = 1/2 x 18.29m x (18.29m/2 x tan(4.76)) = 1/4 x 18.29^2 x tan(4.76) = 6.97 m^2. I would guess someone confused radius and diameter there.
