Easiest way is to mark either the display tank or the sump in 10 gallon increments while filling initially. Then for the water change you can drain the required number of gallons to a toilet or sink and then add the new saltwater.
What I have done is fill a container of the required size on customer site with RO/DI water, add salt mix, and then either run this through an external recirculating pump or just drop a pump down in the bottom and let her run for at least 24 hours to ensure a thorough mix. Drain the required number of gallons from the display or sump, whichever is most convenient, and them immediately refill with the new saltwater.
Dave.M