It looks like a Cal/Mag deficiet.... what do you t...

What I do see in 2nd pic the top leaves are turning light yellow which means your grow light is to close. I would not bet on cal mag with that one spot. When plant is stressed by grow light this can also trigger things that make it look like nutrient deficiency but it's not. just raise grow light 2 inches and monitor leaves closely. If not fixed you'll lose all those leaves when you need them.

To me it just looks like one brown spot.. wherevr it comes from.....What i see is a slightly lightburn in the upper region of your Plant...So keep your Girl happy , and dimm / raise the light only a little. She looks fine keep that track

Hello @DoctorF! Yes it looks like a Calcium deficiency, and Manganese if probably involved too. Do you always buffer your pH? In case you don't, I highly recommend to buy a pH-pen at your local growshop, pH is one of the keys to a successful yield, and a medium with a wrong pH will show random deficiencies or even worst, nutrients lockout. So grab a pH-pen, and eventually an EC-meter to better understand how strong your meals are. Double check the pH-pen calibration (with dedicated buffering solutions) and make sure every watering/feeding is in the correct pH range for soil, which is 6.2~6.5. Nothing above, nothing below! Now for your next feedings, adding some Cal/Mag or epsom salt will help, but you will still need to adjust the pH of your solution. Hope this will help, happy growing!