In ASTM D2239, the ISO equation is shown as:
2S/P = (ID/t) +1
where: S = hydrostatic design stress, (psi)
P = pressure rating, (psi)
ID = avg inside diameter, (in)
t = minimum wall thickness, (in).
From a static equilibrium perspective, I do not see where the "+ 1" comes from. So, I dug out my college fluids text, which suggests the following relationship:
P = S*(2t)/(ID)
This can be rearranged to: 2S/P = )ID/t) (i.e. without the "+ 1")

I Googled the issue and found the Engineers edge website, whose equation (based on pipe OD) is as follows:
P = (2St)/(OD - 2t) (with a Safety Factor included).
This relationship agrees with my college text, as well as my static equilibrium free body diagram.