I think it worth point out that 2 is a factor of 390, so its in my magnification set, but in the case of ZN, 2 is too granular. In fact, 3 is a tad too granular. The usables are 1 2, 3, 5, 6, 10, 13, 15, 26, & 30 (in the 30-or-less range). So, I could use 6 instead of 3, but right now, I'm using 3 because I think, in a low volume instance (like ZN), 6 is too close to 10 & 10 is the main usable in ZN's case. So, I use 3 and 10 to get enough difference to make the resulting chart mean something.
My opinion only, mind you.
IW