The problem with computing any fuel endurance to the nth degree of accuracy is that a) real life rarely reflects the plan to that degree, even if the flight is straightforward, and b) if the plan goes out of the window, due to diversions or other unplanned events, all the initial assumptions (altitude, power setting, wind, climb and descent rates etc.) may be incorrect so the endurance will be what you can achieve, not necessarily what you thought.
I'm not for a minute suggesting that you should set off without a good grasp, and a record, of how much fuel the aircraft has on board, how much you calculate you will need, and what nominal reserves and endurance that represents, but agonising over whether it's 3:08, or 3:11, or 3:06 is frankly rather pointless.
On an IFR flight plan with full fuel I'll put 4:50 as my endurance, but I know I could easily stretch that to 5:15 or more without compromising my cruise speed too much if the chips were down.