As others have already stated. The decision as to whether its a 12 fret neck or a 14 fret neck (or even 13 fret) has already been made. Follow the plans. A 12 fret neck is shorter than a 14 fret neck. You have a neck so the decision is made. Measure it.
At any given scale length, the distance from nut to saddle is always the same. What varies is how much of that scale length protrudes from the body. If your plan is for a 12 fret neck then the neck will protrude from the body to the nut, exactly half of the scale length. I.E. on a 17" scale instrument the neck will be 8 1/2" from the body to the nut. If its a 14 fret instrument the neck will protrude further.
Also the saddle needs to be placed accordingly to maintain the correct scale length.
Don't forget saddle compensation. See,
https://www.stewmac.com/FretCalculator.html
EDIT: Also be careful as to whether the plans are metric or imperial. 17" (Imperial) is still common and many people still use the shorthand name of "17inch" when talking about the scale length of a tenor even though it may have been built in metric. 17" converts to 431.8mm. Many tenors are built to 432mm (metric)
Anthony