"""""""""Huh? You are talking about the 15mm tall "plug" that fills the bottom end, and contains the tapered hole? I think I get the Boolean, but what two solids in the cavity?"""""""

When you cut the "insert hole", the cavities will have little solids inside. It could be a bug with how the Boolean was performed, I'm not sure. Lets just finish it here so we don't get confused.

So, the tapered dims, fit to the insert look like this:





If these are set at the baseline, this leaves 2.3 mm before the cavity is reached.

Would you like to inset the tapered hole some? And, if we inset it, would you like the taper to continue out, so that the 9.22 starts at the inset value and the actual base opening is now larger? Or would you just like the opening to be 9.22, straight to the inset, then perform the taper?

Let me know and I'll finish off the plug hole