Brown spots older fan leaves

Hey Danger! It looks like the start of a phosphorus deficiency (potentially calmag but I reckon P is most likely culprit for your current week) - quite common at this stage as they just start going turbo on flowering and start eating everything including themselves to power them through bud production. Feed based on deficiency prevention rather than feeding to rectify deficiency or you can do more harm than good if you overfeed at this point. Better to prevent further deterioration than try to fix it and M(f)uck it up.