I think my Nomad is easily about 2-3lbs more positive than a stripped down harness. Padded backpad and the shoulder pads are main culprits. Not a problem for me because I still sink without a weightbelt, so it actually helps, but you need to do a proper weight check. I know you have a bunch of dives under your belt, but the majority of divers dive overweighted, and the more weight you take off the better.
Some of them mentioned trim weights. You should be able to trim without any trim weights. With a BP/W I'd just get a weighted STA and be done with it. If you weight yourself properly, there's no reason you should need ditchable weight. In a HOG harness it's easy and fast enough to cut the sucker off with trauma sheers anyway. A 7mm with a 3mm hooded vest shouldn't take that much weight. Not sure what you look like, so if you're normally pretty buoyant then that may explain it, but if you have the Halcyon SS STA and metal cam bands, I can't imagine you needing any more than 2-4 lbs MAX if anything. I dive mine with a HP120, 5mm suit and 5mm hooded vest, and don't wear a weight belt. The 120's and 100's have the same buoyancy characteristics. I sink naturally, and by the end of the dive *300 ish psi* there is no air in the bc and I'm breathing normal. So if you normally float, then 2-4lbs should be all you need.
You'd really be best off to do a full check in a pool if you have access to one. Take the empty ish tank. About 200-300psi lower than you usually surface with. For me that's 200 psi. Have a bunch of 1 lb weights laying around if you have access to them, if not get the smallest weights you have. I'd try to get two 1lb weights and four 2lb weights.
Dump all the air out of the BC and make sure the suit is flooded all the way
Hold the buckle of the weight belt and have your buddy slide weights down there one at a time.
Starting with the 1lb weights.
Exhale all the way, then take a normal breath. If you get your head just submerged you're perfect. This is so you're perfectly neutral for your safety stop at 10-15 feet.
Keep experimenting with this. If anything you should only be a pound heavy at that point. Keep in mind, with 100cf of air, you'll be 7lbs heavy at the beginning of the dive vs the end, hence why you don't weight check at the beginning, and try to do it at a pool if at all possible.
You'll find that as you get used to this whole BP/W deally you'll probably shed a pound or two of lead by this time next year. I reweight myself fairly regularly. I do mine a little weird though. I weight check myself in a bathing suit, then I weight check each of my suits on their own, and know the buoyancy specs of my tank and regs, then do the math. It saves a lot of time in the long run, and that way there's no "did I get all the air out of my BC", or "was my suit flooded" etc etc.
Always makes me that much more of my breathing especially at the stops because if you overbreath you'll rocked up
