It's entirely your choice! If you think that there is only one condition in which the formula can be used, you say 'condition'; if you think there are several conditions, you use the plural 'conditions'. It's as simple as that. By the way, are you sure that 'condition' is the right word? Are you talking about physical conditions (such as heat or humidity)? If not, then 'situation(s)' might be a better word to use.

Please see the revision to my original answer, under 'comments'!