@nprofile1q... @nprofile1q... I wondered if this query might be related to a documented pantograph master size.

‘Midpoint’ implies something halfway, and I wasn’t sure if that was what you meant. Given that optical size modifications accelerate towards the small end, I wonder if one could indeed identify a midpoint in terms of the average of the stem thickness at the smallest and largest sizes?

Applying that calculation to the Sitka family, I end up very close to the nominal Subhead size (which in Sitka targets 16.7pt).