Paul,
I'm no expert on these things, but I think that if you write an original article and include Tinseth's formula along with appropriate references (for example a link to Glenn Tinseth's Hops Page and credit him for developing the formula) you would be on pretty safe ground. It would be much like citing a formula from a physics or chemistry book in another article.
Copying his page or an article someone wrote wholesale would clearly be wrong, but I believe quoting a formula with appropriate references and citation is acceptable.
Cheers!
Brad