I think he means that the glue on the bottom of the knot could make the diameter at the bottom of the knot about 3mm bigger. That would mean that I need a hole size of 25mm
This is very good as I found a forstner bit set of 15-20-25- and 30mm
I think I will try those bits on some softer wood to find out how deep the hole should be when the knot arives..
Maarten