Difficult to know just which bit you need help with, but (in the absence of any further context that might render it unsuitable) I can't help feeling that the literal, straightforward approach is probably best in this instance.

You might well need to look again at the use of 'definitive' (verging on a 'faux ami'), but without knowing more of the context, it's hard to see why they felt the need to express both 'ferme' AND 'définitif' — has there perhaps been some hesitation or reluctance in some quarters?