The last time I told Utabintarbo to change the fin diameter, I said: "I'm increasing the diameter of the fin pattern to 25 mm". ( think I asked him to change it at least 4 times! Sorry Bob!)
Well, that was meant to cover the Barton core, plus 2mm.
Seeing that we now have some more accurate (!) measurements
here, it looks like a 20mm diameter would do just fine.
(Go ahead, calculate it!)
Bob, are you there? (ducking)