As I wrote earlier, Profili will not work with a Mac. I can do the interpolation with an Excel spread sheet (then go back to MacFoil), but was hoping there were some Mac users out there that might have some 'back-of-the-envelope' solutions.
Pay attenction because this method doesn't work well if the wing is tapered AND root and tip airfoils have different thicknesses.
Doing so the wing thickness in the middle part will be thinner then desired.
If you have a Windows emulator on your Mac you can use Profili.
ciao
Stefano