18 is a good starting point, but watch for the scope slipping. I have seen more scopes slip in Spuhr mounts than in all other high end mounts combined, so depending on the recoil of the rifle, you may have to go a little higher eventually. I would not start at 15 with a Spuhr mount. That would likely slip unless it is a 22LR or some 25lbs 6BR.
ILya