water displacement and water change %

Hello everyone I am about to hit my first water change with my saltwater system. I have allways replaced my freshwater tanks with water changes based of total water volume minus the water lost by water displacement "amount lost by rocks, sand or gravel". For example my system has the capabality to hold 103 gallons including the sump and DT but my water displacement loss from rocks and sand is minus 30 gallons of water so the total actual water in my system is 73 gallons. Do you guys do water changes with % based off water displacement or not? 10% of 73 gallons is 7.3 gallons while 10% of 103 is 10.3 gallons. I know the difference is pennies but I am just asking out of pure curiosity.