Why don't we start with the engine and the average fuel consumption?  Then consider what the pilot or passenger can sustain. Very simple , multiply the sustainability by the average fuel consumption. Wola...the amount of fuel needed  plus reserve. Now, what does the aircraft carry? If it is less then previous, enlarge your tanks! For most of us, about 3 hours is about right plus reserve.
Giff Marr
N229GM IV-P