Bouyancy characteristics of a 1st stage? :)

Since I added second 1st stage to my Y valve (single tank, steel), and it weighs almost 0.5kg (470g, that is), would this allow for removing the same amount of lead from my weightbelt? Number of 2nd stages has remained the same.

I reckon 1st stage cannot be + bouyant since they sink like a rock, but do the numbers add up?

I was thinking to remove 0.5 kg from my weightbelt prior to adding another 1st stage, so I'm pretty sure I should do it now (remove 1kg that is).

I asked a similar question on another list regarding fins, and people suggested I use a fish scale (hook style?) to measure their weight while they are immersed in water. I assume you could do the same with your first stages if you want the exact number.

Otherwise I'd just gestimate -0.5 kg (-1 lb.) per 1st stage. I guess the actual negative buoyancy is a little less than the dry weight since the displaced water is heavier than air and because there's a small air pocket inside first stages.

If you just want to determine the right amount of weight to carry, then the best approach imo would be to put on all your gear and almost empty tanks and do a buoyancy check in shallow water, say at 2m depth. Just empty your BC and drysuit (if applicable) and keep dumping small amounts of weight off your belt until you are neutrally buoyant.