If I am adding just a bit of weight to increase the swing weight and power I put it at 12. Most modern frames have plenty of stability already. If I am going for a bigger weight increase I will go for 3and 9 or10 and 2 with counter balance in the handle.
Angell TC95 16x19. About 322 grams with overgrip. I found the ball to fly a bit on the BH side (one-hander, if it matters) and adding about 5 grams total at 3/9 seemed to help. I also have about 4 grams at the bottom ofbthe handle (just slapped some tape on and couldn’t be bothered to measure) and 1.5 grams at the tip, which seems to have helped particularly on serves.