I would try this. Take the 15 gal
of saltwater from the LFS
and put it in a 20 gal
container liek a rubbermaid tub. Then add 1 gal
water. Have a heater in the tub and a PH
to circulate the water around. Check SG
after the RO
water has had a chance to mix with the saltwater for lets say 15-20min and is somewhat close to tank temperature.
I dont think it will take much water to drop the SG
0.001. If after adding 1 gal
water you see no change then try 1 more gal
water and repeat till you find the proper amount. My intial guess is that it will take 1-2gal.