I'm sorry but there is NO
formula to give you the exact water and salt to come up with an exact s.g..
Every salt is different so that is a big problem in itself, and, I've found slight variances when mixing even different pails of the same brand. (I think coarser or finer affects the volume of salt slightly)
Best thing you can do would be to take a given volume of the salt you use, and mix it up to the s.g. you wish to have, and find out how much water it takes to get as close as you want to that s.g..
I do it slightly different and add a certain weight of the salt as measured on my balance scales, and add to a given volume of water.
Of course, you would want to do this for all s.g. ranges you use, or, you could make up the normal batch at the s.g. you most use, and then mark down how much to dilute a given volume of the main s.g. to get any of the other s.g.s that you wish to use.
I would still verify each and every time after mixing though, especially when treating with hyposalinity.