Should be more than enough *LINK*
Derek
>My original numbers were only for example purposes and not intended to be real.
I did not find the numbers for European Beech, which I guess is what you are using (I know you indicated the species at one point), but the link below is a shrinkage calculator and alows selection of various species.
I computed the values for 15" of quarter-sawn American Beech (maybe it is close to European Beech or whatever you are using) and the result for 6%-16% MC change was 15"-15.22". That is on the order of 1/4" change for the 15" slab. American Beech is not a very stable wood so I would expect that 1/2" is plenty if the wood is quarter-sawn (flat sawn on edge is quarter-sawn). THis should be pretty darn worst case since I doubt your wood is still at 6% - my guess is likely closer to 8%-12% or so, but it does depend on the environment it is in and for how long, of course.
The flat-sawn number came out to 15.46" so that is what you are specifying. My opinion, given the information I have, is that 1/2" of clearance should be very safe.
http://www.woodbin.com/calcs/shrinkulator.htm