The relative weight of gas doesn't matter, except for it's contribution to the buoyancy of the diver - so the +/- figures are most telling.
The diver is 'most' negative upon reaching the bottom on descent. This is the point where they have the most gas and the least buoyancy from their wetsuit. If they multi-level upwards and with each breath they take.. they become more buoyant as the dive progresses. In technical diving (where gas consumption is high) that happens quickly.
So (using Luxfer AL80):
4 tanks are -5.6lbs
Carrying weights to compensate for 5/6ths (tanks at 500psi) empty tanks later in the dive -14.6lbs
Total negative buoyancy at this point: -20.2lbs
The other factor is wetsuit buoyancy. This is dependent on wetsuit size (S/M/L/XL etc) and wetsuit thickness (3mm/5mm etc). The loss of buoyancy at depth doesn't matter - it's a decrease, but doesn't contribute towards negative buoyancy in any way. What matters is the amount of weight needed to be worn to compensate for its maximal positive buoyancy (an issue on descent and ascent at shallow depths).
So, to exceed a requirement for 20lbs of compensating weight (
bringing total negative buoyancy beyond the threshold of a 40lb wing), the wetsuit needs to be more than 20lbs/9kgs un-compressed buoyancy.
According to
this chart, 20lbs/9kgs is on the outside limits for wetsuit buoyancy. More buoyant than 7mm with over-vest and hood....
In the study/article linked above, they measured and recorded that a 5mm suit only provided 2.63kg/5.8lbs!
Also add to that consideration the total loss of buoyancy from the suit - which is an easily calculated fraction, based on ambient pressure.
DEPTH - POSITIVE BUOYANCY
Surface - 20lbs (un-compressed)
10m - 10lbs (50% compressed)
20m - 6.6lbs (66% compressed)
30m - 5lbs (75% compressed)
40m - 4lbs (80% compressed)
50m - 3.3lbs (84% compressed)
60m - 2.85lbs
70m - 2.5lbs
80m - 2.2lbs
90m - 2lbs
100m - 1.8lbs
That positive buoyancy needs to be factored into the overall calculation - subtracted from the total negative buoyancy. Realistically, on all but the deepest trimix dives, a 20lbs suit doesn't need 20lbs of compensating weight. For the 'average' tech diver (up to ER certification), they would always retain a significant amount of positive buoyancy from their wetsuit (retaining ~15% positive buoyancy)..which is noteworthy when considering the overall premise that 40lbs of bladder lift is sufficient.
Bear in mind also, that when calculating surface flotation requirements, only the weight of gas is a factor - the wetsuit is uncompressed and balanced against the weight carried to offset its buoyancy loss at depth.
So, to continue the calculations:
Total negative buoyancy at this point: -20.2lbs
Total weight needed to offset a 5mm wetsuit: -5.8lbs
Less retained buoyancy in the suit (@50m): 0.96lbs
TOTAL compensation required from wing bladder: 25.04lbs
Add to that, consideration of respiration derived buoyancy/compensation:
Average tidal volume of diver (respiration) : +/- 3.3lbs (buoyancy variation upon normal breathing)
Average vital capacity of diver (respiration) : +/- 8.25lbs (buoyancy variation upon maximal breathing)
END RESULT:
A technical diver with 4x Luxfer AL80, in a 5mm wetsuit, diving to 50m, needs a maximum buoyancy provision of 25.04lbs, of which ~4-5lbs can be offset in the short/medium term by a deeper breathing pattern.