I haven't done anything with fiberglass pipe before, but I looked at the formula to see what it would take. You would need to re-arrange the appropriate equation in III-2.2.2(A) to solve for the pressure (P). Which equation to use depends on the type of fiberglass. For example, in equation III-2.2.2(A)(A.2):
t = D ÷ [(2Sb ÷ P) + 1]
Rearranged for P it would be:
P = 2Sb ÷ [(D ÷ t) - 1]
The value for Sb comes from Table III-4.2.2 in the last column titled "Design Stress".