Cal/mag is usually calicum nitrate so it has alot of N (4-0-0) so you might want to steer clear of adding more in flowering, you need like a 1-0-0 version of cal/mag if you must add. The majority of the plant growth is done so less demand for nut's and what the plant don't want builds up in your soil and screws with ph. FFOF is very rich soil and your plant doesn't need it all anymore and any ph below 6.2 lockes out calicum, maybe change ph of water between 6.8-7.0 .
I myself don't bother with cal/mag in flower once the stretch is done.
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@Backlipslide said the plant will start canabalizing nut's it needs at this point in flower.
Adding of nut's during flowering means you have to remove another nut because of ppm or risk burn(not good in flower), your in FFOF soil you may risk more problems at this point in flower trying to add cal/mag.