hahaha.....someone customiziing his racquet....hehehe. Anyway, the thing to remember about adding lead tape is:
1. Sweetspot moves in the direction of the added weight.So if you add weight at 12 o'clock, your sweetspot will move up.
2. Adding weight to anywhere along the hoop will change static weight, swing weight and sweet spot position.
3. Added weight does stabilize the racquet on off centre hits and increase "plow" through.But depending on your strength , it will tire you out faster and slow your racquet head speed down.
4. I think most pro's like to add lead at 12 o'clock. Improves the serve strength as well. Counter balancing is not necessary, but I like to do so if weight is added to 12,2,10,3,9 o'clock position coz i like to keep my head light balance the same or close to it. Only the 6 o'clock position or racquet throat hardly changes the balance and swing weight, so I don't usually bother.
5. You may customize as much as you like by adding weight at all positions, but it's all trial and error. See where you are contacting the ball more when you hit ( look for where the ball fuzz is mainly located) , before deciding where you would like to add the weight.
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I am not sure why adding the extra 4g on top of the previous 4g you added to the pdrgt caused the "twang" sound again. I only got to try hitting it when you added the first 4g and it felt much more stable. Could be it became a little too heavy, making you hit off centre more because your swing hasn't adjusted to the new weight ? Not sure ,bro.

i guess it's more a trial & error kinda thing. will try putting some on the 2-10 o'clock n probably this time .. put a bit on the handle to maintain the head-light feeling.