Most people do approximately the same volume in a water change most of the time. Drain out how much water you normally do, say 20 gallons. Measure the outgoing water in the same containters from which you will be drawing the incoming water so that you come up with equal amounts. Once you've drained the appropriate amount, draw a line with a permanent marker on the sump or side of the tank. Next time you need to do a water change, hook up a powerhead to a long hose (long enough to reach somewhere that won't be affected by the old saltwater, like the driveway) turn on the pump and let it run until it goes down to the mark. Then plop that powerhead and hose in the buckets of new SW and pump into the tank.
Works for me and I can do a water change lickety-split this way.