For growing in soil, checking ph and EC/PPM is dead useful, but optional, you can do without it and get good results. For growing in coco, checking and adjusting pH and measuring EC/PPM is a must. Those pictures look like you had a burn, due to overfeeding. Calcium deficiencies show on the newer leaves, not the old / lower ones, so it's not that (and it takes weeks to show) and it looks like red spots on the leaf, magnesium deficiency look like yellowing between the veins of the leafs. pH for coco should be in the range of 5.5 to 6.5, ideally you want to be at 5.8
HI @theNoyd! Not so close to me, I see this plant suffered from nutrients toxicity, so I think she will need at least 2 more weeks, maybe 3. Since you're going to have only 1 cola, I would wait for this cola to be perfectly ripe. The best way to tell when your plant is ready for harvest is to grab a jewelry magnifier / hand-held microscope, and take a daily look at the trichomes. The trichomes onto the buds should have changed from thin & clear / transparent to fat & milky / cloudy / opaque. Once you'll get only milky trichomes, the harvest window is open! For an energetic high, harvest before the first amber trichomes appear. For a couch-locking high, you'd rather harvest once 20-30% of the trichomes have turned amber/red, you'll get more CBN/CBD but less THC. Hope this will help, keep us up-to-date and enjoy your harvest 👊
