rust spots in the middle of leaves

im leaning towards a calcium def, but i would like confirmation before i try to fix it so i do not create another problem.

Ebb and grow with botanicare pure blend. This pics are from day 4 in flower after a 48 hr dark period. EC just under 1, but my mg:ca ratio is about 3:2 and i am using RO water. ppm is falling slightly and ph is rising slightly daily. ph started at 5.6 4 days ago and now its at 5.9

New growth looks fine, but middle-aged leaves are forming tiny rust spots. Not the large rust spots i see in most pics.

I agree with your original diagnosis.

What's your temps and rh.

Check your ph before you add anything more nutes.

room temps are 80-82 during the day and 72-74 at night.
Res temps are 64-65.
rh is 45-53 (i have some plants in that room at week 7)