You're thinking about the right stuff.
The biggest unknown about your situation, pointed out by others, is that looking at the weight of the rig only is irrelevant except for the entirely different situation of needing to be sure your wing will support your rig when you aren't in it so you can ditch and don at the surface. That isn't the question of a "balanced rig."
What matters 99% of the time is the buoyancy of the entire system, including the diver. You are buoyant. My steel BP and AL cylinder, like yours, is significantly negative by itself. But, my body is significantly buoyant (more than it should be, unfortunately). Together, we make a balanced rig.
Separately, a comment on some of the other feedback:
Let's assume you are in fact -15 even when accounting for your body. I'll take issue with the notion that a rig that leaves you (you + rig) at -15 is okay. That's sheer nonsense. I'll say right now that bringing up 15 pounds net negative is horribly unsafe. You cannot sustain that for long at all, particularly if the reason you can't fill the wing is that you've gone OOG and also don't have anything to breath. See how many laps you can swim holding not just 15lbs, but enough lead to make you, overall, -15. My bet is maybe 1 or 2.
The other reason that -15, or -10, or -5 is undesirable is that it requires you to have significant gas in your wing to carry all that weight. That volume of air in your wing requires constant BCD adjustments as you change depth and creating more drag.
The goal of a balanced rig is not just so you can swim it up, but so you can do much of the dive with little to no gas in your wing. In a single and wearing a rashguard, after I get below about 1500 psi, I have zero gas in my wing for the remainder of the dive. At most, at the beginning I have a couple of pounds worth of gas in there - almost nothing. If I were in a wetsuit, I'd carry a tad more weight so I could hold my stops, but the goal is to have as little gas in the wing during the dive as possible.