Hmm.... don't you have a reserve on the tank? Years ago, I was going 70 on the interstate when the bike started sputtering... and had that sinking feeling when I realized it was
already on reserve from the last time I'd filled it up.
or you could set the trip odometer and get gas by the mileage.
Generally, if you think you might need gas or have to calculate how many miles you think you can go, just get gas now.
Pro tip: If you go to a gas station, don't remove your helmet AND keep one hand in your pocket with your index finger pushing against the inside front of your jacket, you can get gas, smokes and lotto tickets for FREE!