I think of this as three problems -- none of which have anything directly to do with how much weight I use.

Perhaps an example might help. Imagine you have two divers using magic tanks that are equally bouyant full and empty. Both dive in trunks and a t-shirt (so no suit bouyancy). One is naturually very bouyant and the other is much less so. Diver A uses 20 pounds and diver B uses 4 pounds. Both are properly weighted (neutral with no air in the BC) and well balanced. Why would diver A need more BC lift in a BC than diver B?

IMHO, here are the three questions:

1. Can I float the rig, at the surface and, without me in it, at the heaviest?
You want to be able to take off your rig and have it float. So you can add up the net bouyancy of everything that will be attached to a fully inflated BC (full tank, reg, gauges, integrated weights, can lights, pocket items, etc.). Plan for what you might use down the road in terms of tank size, pony, etc. when you do the math. Personally, I would want to do this using probably less than 90% of the BC lift.

2. Can it adjust for my change in bouyancy under normal conditions?
With a shell drysuit, there should be no change in bouyancy as you go deeper (assuming you properly manage the suit and it is in working order). If it is a neoprene suit, you should account for lost bouyancy as the suit crushes. Then you need to add the difference in bouyancy between a full tank and an empty tank. I can't imagine this gets you anywhere near 30 pounds.

3. Can you get yourself positive in a worst case scenario?
In this case it would be a major drysuit flood with a full tank (at depth if your suit crushes; and at the surface in all cases). The most conservative scenario would be to have 100% of that lift from your BC with a bit of lift to spare. A less conservative approach would be to get to that lift using 100% of your BC plus some fraction of your ditchable weight.

When I dove dry, I opted for an approach that was all of my BC and 1/2 of my weight ditched. If I have a major drysuit failure, I am fine giving some weight to Neptune.

What would the math look like for this? I would first take my personal bouyancy (i.e., swimsuit w/no gear), add that to the bouyancy in my gear, add that to my weight belt. Lets say I am neutral. My gear at its heaviest is -12 and I wear 35 pounds. That means I need to have lift of 47 pounds (I am implicitly assuming that 100% of the suit lift is gone). If I were to dump half of my weight (17 pounds) plus my BC (36 pounds) that would equal 53 pounds -- more than enough.