If someone presents an algorithm which incorporates 27 different formulae, and one of those is incorrect, you could properly say "You're using a wrong formula."
However, if someone is presenting a scheme which involves only one formula, or if the questionable formula of a multiple-formula algorithm is currently under discussion, then you'd say "You're using the wrong formula."
You use "the" when the item of discussion has somehow been identified, "a" when it is anonymous.
(Similarly, if the entities are being discussed in plural, you use "the" if the entities are identified, and no article at all if anonymous: "You're using the wrong formulae" vs "You're using wrong formulae".)
(Pretty sure this has already been discussed several times, and this question is a dupe, but have not had much luck finding dupes of late.)