Estimate cost per refuel.
A fill-up costs the pump price times the volume actually added, and that volume is the tank capacity multiplied by the gauge gain rather than the whole tank. Working from percentages is what makes the estimate usable at the pump, since the gauge is what you can read. If the target level is at or below the current level the gain is floored at zero rather than producing a negative charge, and the range added converts the same volume into the more meaningful question of how far it gets you.
Volume added
Gallons = tank capacity x (target level - current level) / 100
Cost and range
Cost = gallons x pump price; range added = gallons x mpg
Rarely. Most float gauges read optimistically in the upper half and drop faster near empty, so a fill-up computed from the gauge can be off by a gallon or two. Odometer-based tracking is more reliable for budgeting.
The fuel cost is identical. A full tank adds weight — about 6 lb per gallon — which costs a fraction of a percent in economy, while frequent small fills cost time. Neither effect is financially significant.