With the wetsuit and rig I was using in Bonaire, I ended up with about 1/3 of my lead threaded on webbing and mounted right at the shoulder of the cylinder. Moving the weight from my waist to the shoulder of the cylinder worked very well, allowing me to balance out my negative fins.
It's simple physics. The farther north (i.e. headward) of your center of buoyancy you place the weight, the greater torque it produces toward head-down. If you have negative fins, the farther south of your center of buoyancy they are, the more torque they produce toward feet-down. If you go completely quiescent, you will rotate such that your center of weight is directly below your center of buoyancy (which is why anterior placement of weight-belt lead is more stable and tank-mounted ponies tend toward turtling), but as in neutral buoyancy and horseshoes, close counts here.
Placing 1 kg north 20 cm from your center of buoyancy has the same effect as placing 2 kg north 10 cm or placing 4 kg north 5 cm. Most jacket BC trim pockets I've seen are very low and have very little capacity, meaning their turning moment is slight at best. On the other hand, a spare band at the shoulder of the cylinder (or even the old "ankle" weight around the valve) will have a much more pronounced impact. I've found about 1-2 kg at the shoulder of the cylinder is sufficient to *greatly* ease the trim of most of my non-overweighted jacket BC buddies, so I've made up several "trim bands" that they can put on the cylinder no matter what BC they're using.
Now, on the other hand, when I'm diving dry, my center of buoyancy is much farther south than it is when I'm diving wet. (Wet, my buoyancy is mostly in my big ol' lungs and my wing. Dry, it's everywhere, and I can send some extra air to my legs if I need to move my center of buoyancy down a bit more.) When I'm dry, all my weight (save the backplate) goes to the weight belt. When I'm in a 5mm suit, 2/3 goes on the belt and the rest at the shoulder on a trim band. When I'm in a skin, none is on the belt -- I have negative buoyancy south of my lungs, so to be in trim, I must add a small amount of extra weight way up at the shoulder to torque myself around the air in my lungs/BC into level trim.
With my wing, posture can roll the air north and south, providing a nice level of fine control over trim. With my jacket BC's geometry, posture has much less impact on my center of buoyancy, so I have to work harder to dial my trim in more precisely. In the wing, all it takes is halfway decent weight placement, and I can maintain trim with basically no observable effort. In the jacket, weight placement is almost all I have to work with, but if I get it close enough, what little postural control I can exert on the air in the BC can be sufficient. Oh, and in the wing and my drysuit, I can just about fall asleep and still be in precise trim. :biggrin:
