Try 4 lbs in pockets on tank. If that doesn't give you problems, move upward.
I use 8 lbs, with 4 pockets. My pockets are too small for 3 pounders, and it doesn't seem worth the bother to replace them. With weights in pouches, that keeps me from needing weight belt.
if you use a stainless steel backplate, that's good for another 6 lbs of ballast, and then I add another 4 lbs of stainless bolted to that. A steel single tank adapter adds another 2 lbs.
In essence, I have 20 lbs of weight on my back, and it gives me no problems, though it can take a bit to get used to.
You want to make sure nothing can slide around, and all weight is as close to your back as possible.
I've never tried this kind of thing with a BC. If the tank is capable of flipping around, you will have problems. A backplate rig keeps everything very stable.
Bottom line, just see what works, what doesn't, then go from there.