Dear Percy,
The thing about backweighting the grip of a putter is that you need to define your objective. You can change the overall weight of the putter, you can change the swingweight "feel" of the putter, you can change the flex properties of the shaft, you can change the balance point of the putter, and you can change the vibrational node of the putter. Which one do you want to change, by how much, and to achieve what effect?
Apparently, according to clubmaker Dave Tutleman, changing the grip weight has no real effect on the playing characteristics of the putter in terms of torque and moment of inertia. An addition of grip weight does make the swingweight lighter, but that may or may not be a good thing in a putter. Some people like to feel the putterhead, so a heavier swingweight is desired. others would rather feel the putter more balanced in the stroke, so the inertial forces built up in the putter movement don't cause much trouble, but this conflicts with what Tutleman says with regard to backweighting the grip.
The folks at Balance Certified Golf make a butt weighting system, but the objective is not really to change the swingweight (although it does). The purpose is to move the vibrational node in the putter further up the shaft into the center of the hands on the grip. The node is the point in the shaft where conflicting vibrations cancel each other out, so there is a "feeling" of solidity and stability inside the hands at impact that the golfer associates with solid impact. The current location of the node can be identified by striking the sole of the putter's toe on the ground like a tuning fork and running a ballpoint pen up the shaft until the vibrations subside. The usual location of standard spec putters is a couple of inches BELOW the bottom of the grip material (not good). The added weight in the butt moves the node up into the grip material so the hands can locate above it. How much weight depends on where the node starts out. Balance Certified sells a changeable set of three different weights of 10 grams, 30 grams, and 50 grams. They are trying to cover all bases for the various putters out there in golfers' hands.
Adding weight all the way out in the butt of the club has a more dramatic effect than changing the grip weight, so to get the same effect with the grip, you would presumably have to use MORE weight. Even so, with a shorter-than-usual putter (33") and a putterhead closer to heavy than to light (360g), the swingweight won't be that heavy to start with (from the shorter length), so I would suggest trying 10g as a start. Perhaps you can experiment by taping the 10g on the middle of the grip to see how that effects the node's location and swingweight. That way, you won't waste material.
If your objective is to decrease the swingweight so the putterhead is less noticeable, then about 6g of grip weight takes off about 1 swingweight for most lengths, but your's might require 8g/sw. I would try the node experiment and try to assess how you like both changes: the swingweight, and the vibrational node's location.
Obviously, any addition of grip weight will make the overall putter weight heavier, but in my view that is all to the good. A heavier overall weight doesn't really affect the send of the putterhead much or even the tempo / pendulum timing much. But it does stabilize the tendency of the putter to stay moving online, and so promotes accuracy. It also helps remind you to let the arms hang fully in a relaxed way and to leave the arms relaxedly extended during the stroke -- the heavier mass helps pull the arms downward better.
Here are some technical references you might find useful:
The GolfWorks - Backweighting the grip
Google Search: backweighting grip golf
Google Groups: View Thread "backweighting"
Google Groups: View Thread "Standard Club Swingweighting
Golf Component Shop Talk Digest
Clubmakers' Resource - What Does Swingweight Really Mean? by Dave Tutleman, Oct 18, 1998
Swingweight formulae
DaveT's Club Design - Swingweight - What Is It, And Why Do We Care?
DaveT's Club Design - Predicting Swingweight
Balance Certified Golf
Magic Driver #4 - The Balance Certified weighting scheme for moving the Node in the putter
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Cheers!
Geoff Mangum
Putting Theorist and Instructor
The PuttingZone.com
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