Hello Fellow Stressers!!!

What is correct way to calculate the wall thickness of pipe based on design pressure considering C.A & mill tolerance. I have seen two approaches:

1) Approach 1:

t = 0.195 in. (calculated thickness as per 304.1.2 t=PD/[(2*(SEW+PY)]
c = 0.125 in.
tmin = (t+c)/0.875 = 0.366 in (after mill tolerance)

2) Approach 2:

t = 0.195 in. (calculated thickness as per 304.1.2 t=PD/[(2*(SEW+PY)]
c = 0.125 in.
tmin = (1.125*t+c) = 0.344 in (after mill tolerance)

I think approach 1 is correct considering 12.5% mill tolerance, but when I divide 1/0.875 equals 1.143 not 1.125. Please share the correct interpretation for this formula.

Warm Regards,