When I switched to cobs I was aware of the mag def issue. They only ran for about ten days before I started seeing a need for mag, I considered a few options and decided to buy a gallon of calimagic. I think it was
@HydroRed that told me he was using 2ml pg, that's where I started and it cleared up the mag def pretty quick. No issues yet, I'm kinda stringing these plants along right now waiting for clones, so I haven't kicked everything into high gear yet.
To add.... I don't see a need for more calcium in MC, but like I said, the extra bit I'm adding hasn't caused any problems.