I picked up two vintage shave mugs this weekend, and the store owner gave me two old shave brushes, I do not currently have a badger brush and I was able to get the old knot out of one, its nothing special just a no name two tone black and whit handle brush, but I would like to try my hand at restoring it.

With my calipers it measures right at 21mm, and there is a 21 embossed on the inside of the handle after chipping all the old residue away. Some sites say for a proper fan go 2 sizes smaller, so I would need a 19mm knot is this correct? The old knot measured 21mm.