Originally Posted by DaleB
I give it one good solid whack and the dimples appear to be fully formed, well defined, and with a distinct ring around them just like if I'd done them with a squeezer.
The tip to look for the concentric ring around the dimple as proof of it fully formed, is not complete evidence that the dimple is fully formed.
When the dimple is almost but not quite fully formed, the skin will be in contact with the outer perimeter edge of the male die and it will leave a scuffed ring, but the entire flat face of the die will still not have contacted the skin. This contact is what finishes the dimple properly and returns the skin to being completely flat.
So, what you actually need to look for is the concentric ring, with
uniform skin surface scuffing on the entire area within the ring.
If you are a builder that is using a single layer of masking tape on the male die so that you do not leave the scuff mark on the skin (because you are crazy enough that you plan to leave the airplane bare polished later), then you need to just learn to read the reflection of the skin (which I think is the most reliable technique anyway).