How long ago did the spots first start to appear? Do they appear to be getting worse? I could be wrong here but... the markings remind of severe over watering as a seedling but not anymore. The way the markings are just on the top 1/3 of the tips of the leaves. You're coming up to day 20 since sprout right? Was the plant water-logged for about 7-10 days or so possibly? Definitely not a calcium or magnesium deficiency. All the other things you mention (temp, RH, ph, etc) are good.
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they look ok overall. maybe Im worrying for nothing? Started light feed wit CalMag and decreased Big Boom run off 6.7