The brown spots are calcium deficiency and the light color green between the veins in the leaf is a magnesium deficiency, two separate things. When you correct the issue, dont look for the current damaged leaves to heal, but rather the new growth looks normal. Calcium can be hindered by your humidity being too low and the stomata are closed, you need them open so the plant can transpire and move calcium in it. Make sure your environment is in check, then look to solve the nutrient issue. Also, this late in flower the best you can do is keep the bud from getting worse, dont throw a ton of shit at it thinking your going to reverse anything. Get your environment in check and I would keep a few bottled nutes or salts around to keep things like this from happeneing again. Also, the next size pot might help too if all you want to do is water.