Hmm yep, thats probably why the plants are a bit iron deficient. No worries though it isn't too much of a problem, regular CSM+B dosing will keep your plants looking good.
I'd start off with once or twice a week at 0.2 ppm each dose.
Since the plants are currently iron deficient I'd start off by giving a 0.4 ppm dose of CSM+B to let them recover a bit, then you can dose once or twice a week (twice is probably better).
I agree, in general plant symptoms are always better indicators than test kits.