40 lbs (18kg) is all you need for steel 12 litre doubles. These contain about 5kg of gas weight from full to empty. An ali80 contains about 1kg to minus one kg from full to empty — they float when empty.
So 6kg/14 lbs is all you’re compensating for. The rest is floating on the surface.
If diving in a wetsuit there might be a kg or two difference between shallow and deep.
This is why 40 lbs / 18kg is all you need.
Double 100s hold about 16# (7.3kg) of gas. An AL80 is about 6# (2.7kg) of gas. So, more like 10kg, total. Not 6.
So, if you are perfectly weighted, you would be 22# negative, with an empty wing and full tanks.
How much lead are you carrying? (and are you really perfectly weighted?) If your drysuit totally floods, will 40# be enough to get you and all your stuff to the surface without any help from the drysuit? Probably, but do the math.
Personally, I would suggest to the OP to just get a 60# wing and be done buying wings. There is really not a downside to a 60 versus a 40. The difference in streamlining is really negligible (assuming well-designed wings). I've tried half a dozen or so different doubles wings. My favorite is the Dive Rite Classic XT (a 60# wing). It affords me good horizontal trim with no faffing around. Unlike some other name brand wings that made me need a tail weight in order to get good trim.