Some supplementary information on the calculation:

In B31.1 section 102.3.2 B) Displacement Stress Range, the allowable is calculated as S_A = f(1.25*S_c + 0.21*S_h) [equation 1A]

The code goes on to state, when S_h is greater than S_L, the difference can be added to the 0.25*S_h term so the equation becomes S_A = f(1.25*S_c + 1.25*S_h - S_L) [equation 1B]

Since the sustained stress must be less than the hot allowable (i.e. S_L < S_h) the residual allowable not used in the sustained case is (S_h - S_L). Adding this residual back into to eq. 1A gives eq. 1B

S_A = allowable stress range
S_L = sustained stress
S_h = hot allowable
S_c = cold allowable
f = cyclic reduction factor (f = 6/N^0.2 <= 1.0)
N = total number or cycles