Eddie,
when W8JI talks about "lucky/unlucky" mast lengths i read that as luck with regards to folk who don't think about cmc, use any old length mast they happen to have with any old length wire to a ground rod & any old length of coax,
you would be lucky to pick good lengths that have a high common mode impedance / low cmc,
that's not to say its just luck when you try to minimise cmc on a mast by avoiding certain lengths in favour of others depending on it been grounded or isolated,
running willyynilly ground wires can increase cmc just as much as it can decrease cmc,
coax & everything connected to it has the same cmc issue as the mast & its impossible to know its electrical length, often multiple lengths to different ground points through tuners radios equipment wiring & the beer cooler,
you cannot nor do you need to know, you just need to make it a better length if you have cmc on the coax,
i fixed a few cmc issues ( when it was not convenient to take the antenna down to install a choke ) by trimming the coax or adding a long jumper & trimming for a lower cmc length.
proper isolation fixes the whole shebang, lower noise better signals,
unless you get very unlucky with the choke resonating your coax outer braid,
still fixable by trimming coax length.
