Yes Richard I understand you, but for me, this a hugh simplification.
Obviously If you calculate one only sustained case, to be in the safest way you must choose the lowest SH.
But I think that there would be 3 sustained cases and every sustained case compared to corresponding Sh.
According to your opinion, if i do 3 separate calculations (one for T1, two for T2 and three for T3 ), the results of stress obtaineds will be different with yours.