I'll offer up a couple of other thoughts to consider. If I understand correctly, you're diving a 7mm full neoprene suit, backplate/wing and Al80 tank normally, and sometimes dive to 100 ft, and want to make sure you have enough weight to get down. First, you should be weighting yourself to be neutrally buoyant with no air in your wing at your 15 ft safety stop, with a minimum amount of air in your tank, nominally 500 psi. So that means you need about 6 pounds more weight at the end of your dive than you need at the start. I've tried a lot of different ways to get my weighting right, and what I found personally works well is to just take off 6 lbs at the surface with a full tank and completely empty wing, see if I can descent, and if so, keep taking off lead until I can no longer descend, then add 6 more lbs for an 80 cu ft tank. Or descend to a 15 ft bottom, start taking off weight a pound at a time until I start feeling too light, then add the 6 pounds back, plus the 1 that made me slightly too light
If you're diving with a wing, proper technique to get all the air out of the wing to descend also makes a difference. Try leaning back to start your descent from the surface so the wing is below your body and hold the exhaust open until no more air comes out, then descend in your normal position. That way you essentially squeeze all the air out of your wing
For weights, another option to consider is ditchable weight pockets that slide on your 2" waist strap. I have the Dive Rite ones that mount either horizontally or vertically and each side holds 8 pounds. Unless you're way too feet heavy, which is sometimes an indication of being over weighted. The weight pockets are easily removable before swapping tanks, which is easier than having a weighted STA or cam band weight pockets or weighted pouch down the middle of your backplate. If you are at 100 ft and have a catastrophic wing failure, you can still ditch 16 pounds (or just 8 pounds on one side) and safely swim up, instead of an all or none approach.
I've heard the DUI weight and trim system is very comfortable and gives a lot of options for weight placement, but haven't tried one
If you're diving Al80s, two other options are to wrap an ankle weight around the neck of the tank, or get a cam band with a weight pocket and add a few pounds lower on the tank to counteract the way the tank gets bottom buoyant as it gets lower on air