So what happens when his hand feels fine but there's moisture wicking up from a crack he can't see? That's the whole point of the meter... it finds what you can't feel. That 85% reading you got proves it. His way is just guessing, and guessing with someone else's house.

Wait, your meter said 85% RH on a new build? That's a seriously wet slab! Frank's rule might work on old, bone-dry concrete, but that's a whole different story. Skipping the barrier there would have been a total disaster.

My old Roberts 50 series head has never slipped on me, even on thick commercial carpet. The trick is to check the locking pawl for worn teeth before you even hook it to the carpet. I had a cheap import head do the same creep thing, and it was always that little spring inside the lock mechanism being weak. A good used brand name head from a real tool supplier is way better than a new generic one.