[Grady Clay]

Charlie,

Yes, you are correct in measuring the deck (at the outer shelf as you referred.)
The deck should be less that the squish or at least the same. If the deck has greater clearance than the squish, that tends to create a “pocket” for detonation to occur.
The “rule of thumb” 1.25-1.50 mm refers to both. Bruce has good safe numbers there.
I measure the squish perpendicular to the surfaces.

Best,
Grady