Here's the formula:
X = amount of water you'll be adding (i.e., how much in gallons you want to add to change the salinity - the larger the change, the more gallons you should do it in)
Y = total water in your system (not the size of your tank, but the amount of total water)
Z1 = salinity you currently have
Z2 = salinity you want
Z3 = salinity to have in the water change bucket
Plug it into this formula:
Z2 * Y - ((Y-X)*Z1
----------------------- = Z3
X
So, to simplify: If you have 75 gallons total water in your system, want to change it from 1.022 to 1.025, using a 5 gallon water change, substitute all those values in above. Z3 comes out to 1.067. So you'd take 5 gallons out of your tank, and replace it with a well mixed 5 gallon batch raised to 1.067.