Pay attention to what your gauge reads when to begin to fill up, then mentally keep track of how much you put in and after a while you will know how many gallons you have remaining no matter where your gas gauge sits.
I had my last truck for 20+ years and I could tell within .5 gallons how much gas it would take solely based on where the needle was sitting. Used to amaze people sometimes. Pull into the station, look at the gauge and say, "Better get gas; going to need about 16 gallons." <5 minutes later> Pump: "Clunk" Them: "How much did you need?" Me: "15.89 gallons." "How did you know that?" <grin>