For diving aluminium cylinders:
In shallow water, remove all your tanks, empty your wing and breath from the long hose. Add/remove weight until you're perfectly neutral in your kit.
That solves your inherent buoyancy.
For me, it's 1-4lbs depending on the age and lingering compression of a 3mm suit. A 5mm suit is 6-12lbs.
Thereafter, add additional weight to compensate for the potential positive buoyancy of your cylinders.
You can get cylinder buoyancy specifications from online.
I weight based on minimum gas. Obviously, the regulator gives a little extra weight, per tank, which effectively compensates further, if you go below minimum gas. Weight of regs is also to be found online, if needed.
In practice, that's approx an additional 2lb per AL80 tank, or 1lb per AL40, for me.
In my old, haggard 3mm XL suit with 3mm booties, I only needed 4lbs to comfortably weight 2xAL80. The suit is pretty wafer-thin after a few years of intensive tech diving.
I'm currently diving with brand new uncompressed 3mm XL full suit and 5mm booties. It takes 6lbs to weight it properly for 2xAL80.
I also have a nearly-new XL 5mm suit. This now requires 10lbs, but out of the box it needed 12lbs. I'd anticipate the requirement will decrease to ~8lbs after a few dozen deeper technical dives.
