It really isn't as simple as that.
All knots are different
Some knots have a "hard base" extending several millimetres above the top of the plug. If set deeper than their natural loft, then they need to be set in a hole that is at least 2mm larger in diameter, sometimes more.
OK... many folks say to set the loft of your knot at 45mm to 50mm.
I am going to show you that there is a problem.
#1 rule : Always make the hole for the knot size and loft that you want to end up with.
In other words, start with the particular knot you are using, and make the hole fit IT
#2 rule: All knots are different. Which is why we obey rule #1
I am going to provide you with an illustration.
Here we have two "22mm" brush knots. (from different sources)
Both have the same total height (66mm), both have the same plug depth (8mm).
One (A) has a plug diameter of 22.5mm - that isn't going to even fit into your 22mm bored hole in the handle
The other (B) has a plug diameter of 21.5mm - that will fit into your 22mm bored hole.
BUT
(see, that is a very big BUT, and its in red too)
If you want to end up with a loft of 50mm, then your hole needs to be 16mm deep.
And THAT is where you will run into trouble.
I measured the diameter of those two knots at the 16mm depth, with the hairs squashed in the digital calipers as tight as they would go. In other words.... the diameter that your hole would need to be.
Brush A: 25mm
Brush B: 22.9mm (let's say 23mm)
Sometimes the answer may be to bore a stepped hole for the knot.
Now, the total height of your knot may be different. But the same principles apply!
Rod