I would like to politely differ with this. It's a pretty good rule for thin wetsuits, but begins to break down in colder water.The amount of ditchable weight to carry on your person should be equal to the difference of weight in the tank between full and empty, which will be roughly 6 lbs. All the other weight should be permanently affixed to your person in some other way
In my Fundamentals class, I learned that a couple of GUE divers had done an experiment with a 7 mil wetsuit (probably quite a new one, but still . . . ) where they took the suit to 100 feet and figured out how much lift it had lost. The answer was a stunning 23 pounds! Therefore, if you left the surface 6 pounds negative (the gas you intended to use), you were going to be THIRTY pounds negative at 100 feet. This hugely impacts the amount of lift you need in your wing, but also the amount of ditchable weight you need to ensure you can get to the surface with a disabled wing.
I don't know how much I can swim up . . . I know I can swim up 10 lbs, because I have done it. I have seen a video of someone swimming up 23 pounds. But at some point, one cannot swim up from being too negative, and the ditchable weight needs to be calculated with that in mind.
Again, this is why GUE recommends against deep diving in thick wetsuits.