I always forget if the reserve is 1 or 2 gallons and if reserve is in Addition to the 26 or part of it. I hope I never have to find out the hard way
The fuel tank capacity is "about" 26.4 gallons, which includes the reserve of "about" 2.8 gallons. Meaning, the orange triangle lights up when you have "about" 2.8 gallons remaining.
It's safe to count on 50 miles remaining when the light comes on. The sensor for that is at the rear of the tank, so if you are low and heading down a long hill, or parked facing downward on a ramp while getting loaded or unloaded, the light will come on a gallon of two sooner than if you are on level ground.
Once you have any black showing between the bottom white block and the needle, you are "about" empty and if you go more than 10 or 12 miles you'll be running on the fuel that's in the fuel lines and filter, and can expect to put in 26.5 or more gallons, depending on if you stop when the pump shuts off or if you add a little more to top it off. 26.8something is my record.
Running it down to the orange triangle fairly often will certainly help keep the tank cleaned out of algae and moisture, and regularly adding a fuel treatment like Power Service helps.