Mac,
Learned what I know working with my husband, professional tile
installation, all applications, in Florida.

1) Use mastic, not thinset to set 1/2" durock (permabase). Butt sheets
up, their slight variance will give you all tolerance required.

2) we typically use 1-1/4" coarse thread drywall screws about every 8
-10 inches to further secure the rock.

3) get out of the tech sheets and go to the local dealer who sells to
the pros, (ie: Wholesale tile stores). If you see anyone in an orange
apron RUN!!!