Half an inch will lighten the SW by about 3 points. Can vary slightly based on the shaft weight and if it's counterbalanced. Midsize grip will also lighten the SW.
Basic rule is 4 grams in the grip = 1 SW point, 2 grams in the head = 1 SW point.
Could also go with a lighter grip like a midsize Winn which are usually significantly lighter than pure rubber. Assume if you are using a standard size rubber grip, it weighs 50g. If after all that the head feels too light I would recommend using lead tape first to figure out what feels right. Remember, once you HotMelt, there is no going back. Once you find the right feel and if you don't like the tape look, a decent club maker can replicate with either HotMelt or installing a tip weight. Good Luck.