stage and deco bottles are not factored into weighting as they are ditchable. They are also right about neutral when emptied to normal pressures. You'd only have to ditch them if they were sucked dry
In the ocean, stages (if bottom gas) are usually sucked dry. If deco gas they could be emptied if you are forced to share deco gas. If you have to share bottom gas up to a deco gas switch, then have some elevated consumption cause the whole dive has already gone sideways, the last thing you need it to be is too light as well.
Just a question:
How much weight should be added against each additional stage AL80 ?
Also, I use 8 kg weight with wetsuit as BM in salt water.
How much weight should I have if Turing to sidemount XDeep stealth tec with 2 stages AL80?
Don't guess.
Put on ALL your cylinders with them all near empty (20-40bar) and add (or subtract) lead until you are neutral in shallow water (where your wetsuit is least compressed)