Best thing about a hydro set-up is the plants react very quickly to any changes in nutrients. You said you just went into flower maybe your plants are just getting used to less nitrogen. That’s why they are a little yellow.

As far as ppm’s I would think 1200 ppms should include everything in there.
In other words add everything find your total ppms, say 1500 then dilute it with water to 1200 ppms. Then balance the ph.

Hope that helps.

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