If you have an "envelope" then why not operate to its limits? If it is not safe and sensible then change the envelope. Those ex military amongst us will remember the story...... Boss says turn up on parade at 1400. Adj just to be sure, turn up at 1350. Fly cdr, just tobesure 1340, sgt just to be sure 1330..... Ad nauseum.
If a SAFE limit is 2hrs, then it is SAFE to fly to 1:59 EVERY time