This is my trick for long flights;
First leg, 2 hours, refuel and calculate fuel burn
Second leg 3 hrs, same power setting and leaning technique, refuel and calculate fuel burn.
Should should now have a good average.
Let's say that it's 7.6 gallons/hr, round up to 8 and you can now safely fly long legs and have an hour reserve.
Keep in mind that you absolutely positively have to use the same technique's as in power settings and leaning to make this work.
I have (safely) flown 5 hr legs in a Pa 28-161 with 48 gallon tanks using this trick.
Same altitude, same everything, same day even.....
Keep in mind that fuel gauges will look pretty scary (empty) when you do this.