To flush or not to flush?

Hi Gromies! I’m concerned my gals have a magnesium and/or nitrogen deficiency. The 2 I’m worried about are in flower so I know the needs change around this time. They’re all in FFOF and I’m using the FF trio for nutrients.

Checked their ppms and the one just entering preflower was pretty high but the others were good. I watered to run off for the first time today and the ph was low I think for all. It was 5.6 so my plan is to water in with 6.8 to bring it back up. But should I add cal mag as well or get the ph in order first and skip it for now? Is flushing required?

And for the plant with the high ppm… should I flush her to bring them down? If so, how exactly? My guess would be run at least a gal through since she’s in a 5 gal pot? But idk because I’m new lol

Close up of yellow veins