A is the total required compression area at the roof-to-shell junction, in inches2.
And the formula you refer should be:
A = D^2(Pi-0.245 DLR/D^2)/(0.962*Fy*(tanθ))
DLR is the nominal weight of roof plate plus any attached structural, in lbf
DLR/D^2 have units of lbf/ft2, is multiplied by 0.19222 to obtain inch H2O units and the formula has a numerical coefficient of 4/3.14159, so finally DLR/D^2 is multiplied by 0.19222*4/3.14159 which is about 0.245, etc.
The coefficient in denominator should be in fact 8/1.6*0.19222=0.961

Is your question as one pound mass in the acceleration due to Earth's gravity exerts one pound force?