Dear dralban,
Let me address your concerns and points separately.
SETUP
First, it seems to me your posture is a little too far over. I would suspect that this tightens your lower back a bit too much. I appreciate how this posture makes your arms and hands hang nice and heavy, and that is something to be valued and preserved if you stand a little more upright. The real moving part of the stroke is low in the back and mid-waist area, so you don't want that part of the body too tight. Try standing a little taller to relieve pressure on lower back, but keep sense that arms and hands are "heavy" and inactive.
Second, the phrase "straight legged" concerns me a bit. A little flex in the knees makes you more solidly balanced and stable when the upper torso is in motion. When your focus is on keeping the upper torso, head, and eyes in a fairly defined pattern of stable motion, this will require some reactivity in the lower body to keep the upper part where it belongs. So I recommend some flexibility in the knees, and therefore some slight flex at address.
Third, the phrase "eyes over the ball" is not well understood. Whether the eyes are vertically directly above the ball or slightly inside doesn't matter very much; what matters is that the gaze is directed straight out of the plane of the face as when searching an ocean horizon from the shore, rather than having the gaze angled slightly down the cheeks as if reading a book. It is the gaze and not the precise positioning of the eyes in relation to the ball that is responsible for accurate or inaccurate perceptions of target location.
Fourth, the phrase "hands hanging beneath the shoulders" is also misunderstood. Dave Pelz teaches this, but he is mistaken about it. What matters in making a body movement that results in a straight stroke path is how the shoulder frame is moved, and the position of the hands inside, under, or outside a vertical line beneath the shoulders is irrelevant to the straightness. Naturally, the elbows hang striaght below the shoulders, not the hands. people who make sure the hands are hanging below the shoulders usually get the elbows crooked back towards the hip a bit, and this posture is maintained only by steady tension. This posture is a state of excess "play" in the triangle that frequently and subtly lengthens the triangle in the stroke without the golfer being aware of the uncrooking of the elbows. This is especially the case if the golfer "tries" to run the putterhead in a straight line with action of the hands and arms rather than by shoulder action, and also if the golfer accepts the advice to keep the putterhead low during the stroke. Instead, the golfer should let the arms hang naturally, with the elbows straight below the shoulder sockets, and the hands (typically) a little angled forward with hands a little closer to the ball than the elbows. This is a posture that gets the "play" out of the triangle without creating tension by "sticking" the arms straight out. Remember, it is the movement of the shoulder sockets that counts. When the "triangle" stays the same, the arms and hands should not change in any respect, posture-wise or muscle-tension-wise.
Fifth, your phrase "clubhead in the centre, and hands just forward of centre" doesn't metnion ball position. The bottom of the arc of the stroke is usually the centre of the stance, but not always, as having one or the other hand lower on the handle than the other tilts the shoulderframe, and this moves the centre of the stroke slightly forward whtehr the tilt is left-up or right-up. Also, deliberately placing the hands ahead of the centre at address usually also moves the centre of the stroke a little forward. In either case, your ball position ought to be an inch or two further ahead of the bottom of the arc, regardless of the centre of the stance. I recommend complete neutrality of hands and arms, with a grip that levels the shoulderframe and centers the bottom of the stroke arc in the center of the stance, with the ball two inches forward of the center, just inside the left heel (depends on how wide your stance is -- should be about shoulder width for balance). This way, the body very clearly knows where the bottom of the stroke is located -- it is where and when the shoulders get level in the downstroke and the putterhead reaches the middle of the body.
All together, I suspect you would be more at ease and consistent with a little taller stance (more relaxed) so that a putter length of about 33-34 inches would be ok and a lie a touch flatter than 14 degrees. I like 14 degrees though! I'm using a 14 degree Plop putter right now.
TEMPO
As far as the metronome is concerned, I would first try just sending the shoulderframe up to the top of the backstroke and then relaxing, as if a sniper had shot you in the head. Your shoulderframe will drop down to level at its own tempo. Do this some, and then try to "catch' the falling shoulderframe and send it smoothly upward past the bottom of the arc in a follow-thru. You can combine this with inhaling going back to the top of the backstroke, and exhaling as you relax to let the putter and shoulderframe drop and then send the lead shoulder up to a top-of-follow thru finish that matches the height of the top of the backstroke.
The dropping of the shoulderframe and putter under the sole influence of gravity will set your tempo for you. Once you get a sense of this timing, try breathing in on the backstroke and then drop to the bottom and say "two" right at the bottom. When you get this tempo pretty regularized, substitute "lift" for "two" and finish the stroke by lifting the lead shoulder starting right at the bottom. Breathe in going back, relax and drop and say "lift" right at the bottom and send the lead shoulder up vertically. After you build this pattern, your takeaway should be done to match the total time from top of backstroke to top of thru-stroke. Then the takeaway to the top of backstroke is done to "one potato".
If you need to work with a metronome, I would suggest trying out a setting of about 65 beats per minute (perhaps 70) at first and then work at getting the relaxation smoother and the upstroke past the bottom more matching the free-fall. As you do this, I would bet that this will further slow your stroke down to about 60-65 beats.
In any event, forget "hitting" the ball or even wondering whether your stroke has sufficient power. just practice a smooth drop and lift on both sides of the bottom with impeccable timing.
Let me know how it's going.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
PuttingZone.com
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