Calculating Salinity for water changes

[kylevan]

So I do 10 gallon water changes weekly. I'm running a Red Sea Reefer 250, 64 gallon total I like my Salinity around 1.025-1.026 I'm using instant ocean reef crystals. 1/2c per 1 gallon makes a specific gravity of 1.021 so when I do my water changes I follow these direction and end up lowering my Salinity to roughly 1.022-23 and end up having to ATO with saltwater until I reach my desired Salinity. It gets pretty annoying doing this after a while so I was wondering if anyone has figured a way to get the mix down to where they are able to get their Salinity to the desired level after the water change without having to top off with saltwater.