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Once you get the Knot in hand you can see how it fits and what you need to do to get it set... I would also guess 28mm **From the info given**

I wouldn't really do anything to the brush until the...

You need Metric measurements...

the sizes you are going to be finding are 24 25 26 28 or 30 mm so you want to get as close to one of those as possible..

Also those sizes are NOT exact there is...

Looks like a Half Dollar is 30.61mm so the knot size is going to be slighly less from what I am seeing there... I would double check the measurement with a set of Calipers and order one from TGN...