It will also depend on the wood. Ebony is quite dense so an ebony fingerboard will be heavier at same thickness than those before made of most other materials. I found this very noticeable when I visited the KoAloha factory and they had piles of koa and ebony fingerboards side by side.
I never thought the thickness of the fretboard was particularly important. Now the thickness of the neck + the fretboard - that is important. Keep in mind that the thickness of the fretboard has nothing to do with the height of the action. That is set by nut/saddle height. My fretboards are about a 1/4 thick or 6+ mm. After a little sanding a bit less than that.
I would not use one that thick if I wanted to maintain a 15mm thick neck at the first fret it would make the neck very weak at the nut at 8.6mm. I took these dimensions from a genuine 30s Martin Concert showing the fretbord at .115 inch thick = 2.9mm for a tenor I would go for 3.25mm at the very most 61090962-0989-48BA-BAA2-C56F36D15D09 by Ken Timms,
A very good point Timbuck. On the 2 plans I have, which seem to scale accurately, it measures about 4 mm, so 3.25 would be close. I was afraid that too thin, but I guess not. Thanks loads for the info.����
Thank you all very much for your responses. I will be working on the neck and fret board later next week. My ticker is giving me a hard time. Only running on 3 cylinders. Just have to take it easy for awhile.