M/L propellants are very forgiving and I'd say to stick to 5- Grn. increments (by volume). There is always a Min - Optimum - Max. Obviously you want the optimum and seriously doubt that it is going to the 120gns. I could guess but it really takes "your" range-time to determine this. Perhaps the folks at BH-209 can give you some suggestions. I know I have seen optimum loads published for BP and Pyrodex. .....
Be Safe !!!