It doesnt matter what the tank capacity is.. only thing that counts is what it takes to fill to nozzle shutoff.
When I changed my pump {4dr}, I had 2 gals in tank, plus added one = 3. Gauge showed dead middle of E red mark. pumped 14.7 to shutoff.

Also always refill at the same station same direction.. because of apron slope with rear filler.. and even pump if possible.

On long trip just write down gallons pumped during trip, start and end at same place.