Kim, I'll put one together for you when I come back from the old world
Brad, with my water volume, if I did the math well, my salinity would go down from 1.026 to 1.025 if I pour 12.5 gallons of freshwater and I take off the same amount of saltwater. So 7 gallons aren't an issue.
And it will take some time to mix up because the freshwater is a few degrees lower than the one in the tank. So it will stay in the bottom of the sump for a while.
But I have to double check the math.. not sure if I did it well