wfcm commentary 2001ed what is the formula
 

wfcm commentary 2001ed "what is the formula?"
on page 43 of the commentary to wfcm, in footnote 2 they make a jump from p=51.4 psf to p=85.65 psf. the equation that is used is not mentioned and only values are used: p=[51.4 psf]/[(cos26.6))(cos(26.6))/(16 in./12)]. so the formula is p(new value) = p(old value)/cos^2. they went from the assumption of a horizontal plane projection to a sloped projection (the original roof) and a force normal to that slope. my trig is a little rusty and i am lost as to why this equation.
what formula are they using and where did it come from?
any help would be appreciated, thanks.