Small brown / bronze spots forming on middle leaves

It can be calcium deficiency if your feed is too hot or if you have too much P (if you've added a bloom booster already, this is probable). Both can lockout calcium and magnesium. Potassium deficiency is also possible but not likely. You want to start with the pH and EC of your runoff to make sure you're not running too hot. 800ppms, is probably as high as you want your feed right now. Assuming that's good, see if the problem spreads. If not, then there's no longer any issue.

Calcium is a imobile nutrient if it was calcium deficiency and you fix it, it will only show up as no symptoms on new folliage, the affected leaves will stay sd they are.