That's a strange way of putting it, and numerically incorrect. The installation of tip tanks increases the MTWA from 3600 lb to 3725 lb, with the condition that any weight in excess of 3600 lb is fuel in the tip tanks. So in essence you can have 21 USG (125 lb) in the tip tanks without having any impact on the passenger and baggage load.
Now that is 1.5 hours at normal cruise power settings.
Thus one could use that fuel for a 1 hour diversion at long range cruise and a 45 minute hold at max endurance power all including contingency.
That leaves all the rest of the fuel you carry to get you to destination.
There are very few such aircraft where the limiting factor is not the endurance of your bladder!
Regards,
DFC