The answer for our BCP-16's is zero. The original design used a butt stack of M6--- and was rated for 1 madc at 400H. Our BCP-16's do not even have the butt stack--- we "close" up the gap significantly with the stacking technique that we use--- so, that, they are really designed and require the use of a blocking cap in front of them.
We do build some of these for an OEM of ours with the butt stack. If you ever want a pair with this small, small air gap let me know and I could put you in the next run I do.
MSL