Inside of the frame or out side of the insert. I think thats agreed upon. (I think)
Assuming thats correct what about recoil guides? Inside of the frame? I'll email Rec again and ask again. The last time they said "we dont establish the guidelines we just follow the guidelines" but didnt answer the question.
I also asked them why the tiptop is english (fractional) and the guides are metric. Buy the tone in the email reply they didnt want to discuss it at all. You have to admit that even using fractional referenceis lame like 5.5/64th ? .0859" is more exact.
But should I be measuring the inside of the frame on a rec guide. There is no insert.

Think NOMINAL DIAMETER for wire guides. The difference between the OD and the ID for a light wire guide like a small Recoil is usually just ~1mm. Not until you get to large surf guides or boat guides or Carbaloy is there a difference of ~3-4mm. Think NOMINAL = OD for the larger WIRE rings.

For ceramic rings, the NOMINAL DIAMETER = the FRAME DIAMETER, which is slightly larger than the RING OD. -Cliff Hall+++

Here is a LINK for the FUJI Line Guide "Ring Size Chart",
with Diameters. ... It is a one-page *.PDF File that has ring sizes from 2.5mm up to 50mm, for all ring & flange styles, and lists OD, ID and WIDTH.
-Cliff Hall+++ Merry Christmas

FUJI Ring SIZE Chart is availble at: [www.anglersresource.net]
[www.anglersresource.net]

FUJI Ring HEIGHTS can be found at: [www.fujitackle.com]
[www.fujitackle.com]

thank you Mr Cliff!