Hi Larry, it might be simpler if you could work just with solids. Then a boolean difference should automatically do some of the things you're trying to do manually like determine extrusion distance.

If you want to limit the cut to one wall then you could use subset booleans. That's when you have faces selected on the object to be cut in boolean difference. It will only intersect with those faces.

So for example if you have a face sub-selection on a solid like this:



Then when you do a boolean difference you'll get a result like this:


- Michael