Reach4: Here is another number to throw into your math: the specific gravity of saturated brine is about 1.18. . . .
From an operational point of view, you will do better with an empirical approach. Get yourself a good hardness test, such as the Hach 5B, you can try reducing the brine fill a bit, and monitor the hardness toward the end of the cycle. There are soap-based tests too, but the Hach 5B is better at giving you a 1-drop/grain titration. You can trigger a manual regen if hardness gets higher early. But this experiment will cancel out the variations of things like the resin, whether your system delivers 0.50 or 0.54 or 0.48 GPM during brine refill. So if you take on optimizing salt use vs effective softening, that measuring will be effective at tuning reality. Now that test will probably cost the same as maybe 250 pounds of salt, so the payoff may take a while. But it doubles as a cheap hobby. :-) Be glad you are not paying yourself by the hour.