There is not an exact formula because of several factors. Grip weight is one of them. Weight distribution of the shaft is another. Even the head size and shape can play a small factor due to the weigh the head weight is distributed. Generally, it is around 10 swing weight points difference with a standard 50 gram grip. I hesitate to say that because there are so many variables, especially with the variety of grip weights available. The basics, as you know, are that 4 grams of weight on the grip end of the club is one swing weight and 2 grams on the head end is one swing weight. For every 9 grams of change in shaft weight the swing weight can change by 1 swing weight. Of course, the weight of the tape has a minor affect as well. It is much easier, in my opinion, to use something like our grip tray that goes with our SWRM swing weight scale. Allows me to cut the club to length, put it on the scale and then set the grip in the tray above the butt of the grip. This will give me a pretty good idea o what the swing weight is going to be. Personally, I will cut the shafts, prep the heads, and dry fit everything on the scale before assembly. That way I know if I will need to add any weight to head end (with tip weights, head weights if applicable, or lead tape etc.).