For filling kegs, why not just use the weight? You have to correct for the specific gravity of the beer you're filling the keg with. Multiply the final gravity by the weight/gallon of water. Example:
light beer with a FG of 1.008: 8.345 x 1.008 = 8.422 lb/gallon => A keg with a net weight (total - tare) of 131.5 lb is 131.5 / 8.422 = 15.614 gallons
really big beer with a FG of 1.030: 8.345 x 1.030 = 8.595 lb/gallon => A keg with a net weight (total - tare) of 131.5 lb is 131.5 / 8.595 = 15.300 gallons
If you permanently mark the tare weight on each keg, you can use the marked value, rather than weighing and recording it. But weighing it empty should give you a good idea of whether the scale has drifted, or the keg is not completely empty.
Regards,
Mike Sharp