Originally posted by FissionChips:

It's also not infinitely variable, such that it is possible to get a mismatch where no matter what you do building pieces and terrain just won't play ball.

Ahah! That didn't occur to me, as it has worked in some places, but not in others. I knew I was missing something basic.

You are basically saying there is an invisible 'grid' and the level tool can only snap to grid levels, not between them, so if the height of my floor tiles is between those 'grid points' it won't be perfectly level?
I have found where that occurs my choices are a bit too low or a bit too high, and never 'quite right'....so I guess I should have figured it out. Thank you so much for explaining it!

Thank you to everyone else in the thread for also providing answers to everything. It is appreciated.