i think the rule of thumb for EDK with a tag line is a difference less or equal to 2 mm
i personally use a overhand with an extra pass through not sure the name but its a 2 strand version of: http://en.wikipedia.org/wiki/Double_overhand_knot
same thing 2mm difference in diameter or less
i find it neater than 2 overhands on top of each other and the risk of the not rolling is much less than a single overhand
is that the not you are talking about brenta?