How much tungsten powder would i need to insert in the graphite shaft of my putter?
Depends on how much you want to increase the head weight / swing weight. 2 grams of weight equals 1 swing weight at the head end. The fact that you are adding weight in the shaft changes the net increase slightly since the weight is going further up towards the fulcrum point of the swing weight scale (towards the grip), but generally you can can calculate based on the 2 gram = 1 swing weight. If you have a target swing weight, you should be able to calculate how much weight you need to add.