Yes, you can use (some) cigarette holders as stems, but not the slim versions which are designed for the gentle sex.
The “regular” cigarette holders are, like DV stems, 8mm ID.
Glass holders/chillums on the other hand, have a slight variance due to its nature.
Grave, for instance makes 12mm tasters. You can buy them singly, or in packets of 10. Of the 10, some might/not fit. The OD is 12mm, the wall is 2mm on each side,
So, 12-2-2 = 8mm. Ideally.
I just go to a brick and mortar store with some tips and ( to the annoyance of the counter person ) ask
For a bunch of glass chillums, and buy the ones that fit.
To answer your question, yes, the caliper measure is that of the “inner”
Diameter of the top of the tip where the 8mm screen goes: ~7.85mm.
The stem ID of a VC where the bottom of the tip ( with its O-rings ) goes, is 8mm.
The OD of a stem is 10mm. Usually
Hope that helps.