discerningbev I see from the site they only go from 44mm to 52mm for that size and type of knot. Mine just happened to get set to 46mm although my order showed the loft to be for 50mm. I'm happy with my brush how it came out.
It really depends on what it is that you're looking for. A flat top will always feel larger than its knot size due to no shape keeping the knot in place. My experience shows that 3-band badger really exaggerated that effect. 2-band helps to avoid that to a large degree as also shown from my experience. My time having the 23mm knot / 45mm loft 3-band D01 Flat top it felt small at the bottom of the knot but extraordinarily large on the face due to the splay. It was between a Kent BLK8 and BLK12 to me. The 2-band D01 feels close to a dry 32mm bulb 2-band D01 knot I have but without going that large in a knot size. The part I like about the flat tops is all of that hair touching the face at once, and feels especially nice for painting motions and how they feel larger than their knot size. Giving advice on the standard knot shapes and sizes is easier as there is more experience there from both the manufacturers and the users.
My personal experience with a flat top is to err just slightly lower than what you think is needed. I'm used to fans typically being 2mm lower than a comparable bulb. For my limited experience I would err 4mm lower than what my experience with other brushes tell me. For how happy I am with this flat top being at 46mm vs. the ordered loft of 50mm there seems to be some merit there. Alas it is more than just loft sizes as my experience with 3-band D01 vs 2-band D01 has shown too. Small changes can make quite a difference.
I think that a knot size of 46mm would be quite safe to go with like mine. That is the same loft as my knot has. While mine is larger, it is only slight, and flat tops don't follow the regular conventions that fan knots and bulb knots do. 48mm if you want a touch more flow and openness, and 44mm if you want a more compact knot that has a touch more face sensation to it. That's my assessment thinking on it compared to the brush I have now plus old brushes I've had and brushes I have now and what I think such a brush would exhibit. I unfortunately only have experience with two flat tops, but do realize it can help as it's not a common type and any experiences can help.
Checking with Bernd is an excellent idea as well, but one I think you already know about.
CCity I imagine there are several that would help you with this such as Rudy Vey, Envy Shave, or other vendors if you explain the situation. Have you ever considered trying it yourself, though? I've mostly used 2-part epoxy for what I have put in myself and have had no problems, but others recommend silicone sealant I believe it is for a non-permanent solution for setting brushes. You don't need a great deal of 2-part epoxy. I see recommendations from others to rough up the inside of the bore hole that holds the brush with sand paper, and even the knot a bit, then apply a bit of epoxy and set it inside the brush. You really don't need much, and just a light touch of epoxy to the bottom of the glue plug of the knot has done it for me.
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