Calcium deficiency...something else?

Eh up,
HSO Sour Diesel auto at about 7 weeks, im getting some rusty coloured spots on the larger fan leaves. Seems to be affecting the middle section if that helps the diagnosis!
Had a look online and from the images ive seen it looks like a light calcium deficiency to me.
Can anyone confirm??
Thanks!!

My guess Magnesium deficiency.

Thanks. Ill stick some CalMag inmy DWC and see how she does!