Your formula looks right, except possibly for one thing: Suppose you're a really large creature, but with lower than average strength. Your formula will result in the negative points you should be getting being reduced by the same percentage that the positive points you would be spending would've been reduced by if you had greater than average strength.

Now, I don't know what the rule is, but it seems to me that, if you're larger than human normal size, and yet have lower than average strength, you should be getting back at least as many points as you would if you were human normal size, if not more.