John,
I see you are addressing the issues you posted a few posts above...
You'll be happy when you are done...
Your measurements should be in the neighborhood... But the thickness can vary in different areas so you might see something different at the mast base area.
In any case - don't be shy. You already have a failure there... So removing the glass in that area will likely be necessary. just tear into it! Then reconstruct that area to the correct dimensions.
When drilling through a cored area; you'll feel the drill work its way through the top layer of glass and then it will go through the soft balsa like butter. The next hard surface will be the lower layer of glass.so it is easy to know where you are... But as I mentioned, I think that the area beneath your step has already failed, so you'll need to reconstruct. Don't bother reconstructing the core. The area beneath the mast step is small, and a solid base will work best under compression...
Just some thoughts - Good luck!
PS. Pls take a bunch of pictures as the work progresses!