With FFOF you won’t need to feed for the first 4-6 weeks. Sometimes it can get you through veg and then you would only be using your flower nutrients.
I don’t have first hand experience with Black Magic or Flower Power nutes, so I can’t speak from experience on either of those.
Depends on the meter. I calibrate mine once a month or so. Sometimes sooner. I take baseline numbers of what my ph should be at after adding nutes and ph up/down for each stage of growth. When those numbers drift too much I will recalibrate.