In my humble opinion a formala's copyright counts only for the excact same formula. With other words, when you change the formula, even if it is for one tiny piece of it, it is no longer the original and therefore no longer in the copyright of the original owner.
That means that from the moment you tweak it, you will have the copyright, but I'm no law expert and I might be totally wrong on this one, but this is just my opinion.

Cheers,
Vraxor
Webmaster of TerraCon Revolution