Here's my thinking on this subject...art.chick once bubbled...
I understand the desire for nice trim, but is it really true that you should make that much weight non-ditchable? I keep hearing about these divers who are even putting weights in their wetsuits to reduce BC pocket weight. Most of those pockets won't accomodate enough for some divers in full 7mm wetsuits.
So what happens if I have to drop your belt to make you possitively buoyant? It is my own instinct to want all my weight (21lb) ditchable. Occasionally, I wil have 2 lb on ring weights, but I want to know that when the belt drops, I can be rescued without further surprises to my rescuer.
I want enough ditchable to make me positively buoyant, but not so much that its inadvertant loss would result in an uncontrolled ascent to the surface. For me that's 6-10 pounds ditchable. Whether my non-ditchable weight is in the form of a steel tank or trim weights makes no difference really, does it? And as long as I can ditch enough that I'll ascend and float, that's enough.
I should note here that in over thirty years diving, I can count on one hand the times when I've been in a group with someone who actually needed to jettison weight, but I can't even begin to count how many have inadvertantly dropped weight.
Rick