Measure the actual bulkhead. Both aren't ideal, if you can fit it in the hole 2mm less because the bulkhead is slightly smaller I would do that, otherwise yes 2mm more is your only option but make sure you clean the crap out of the glass where the gasket is going, hand tighten, then 1/4 wrench tighten because you will periodically be tightening a bunch to get the gasket to seal. I've always been on the side of putting the gasket in a bit wet and letting it dry too, depends on your time line.