To figure out how much gas you need to get when you have to prepay for the gasoline you mark your odometer reading or set your trip odometer to 0 after filling up the gas tank.
Then figure out your miles per gallon that your car gets and then you figure out how many gallons of gasoline your vehicle used to drive those miles.
For example if your car gets 28 miles per gallon and you drove 28 miles then you would use 1 gallon of gasoline and if you drive 56 miles then you would use 2 gallons of gasoline.
So if your car gets 28 miles per gallon then you would only need to buy 2 gallons of gasoline if you drove only 56 miles.
Or if you cannot figure out your miles per gallon and use your gas gauge as an indicator for the gallons of gasoline you need figure out your tank size and if you have a 14 gallon tank then you likely need to buy 12 gallons of gas once the gas gauge reaches near empty.
You can just pay more in cash or on your debit card then you think you need in gasoline and then go back in and get a refund for the amount you didn't get in gasoline.