Brown spots started appearing on this autoflower f...

No diary = no information. This could be a magnesium excess although that's fairly rare. From the look of the pics, though, you shouldn't be giving them any calmag at this point and may indeed have an excess of magnesium which will cause a lockout of calcium... refer to this article to diagnose your problems yourself since you don't have a diary: https://futureharvest.com/blogs/articles/how-to-detect-deficiencies-and-toxicities-in-cannabis-plants Good luck!

Most rob ph and Calcium issues ... Anyway, for this girl all feed should be stoped. Cal mag can make excess too ! Cal mag has NPK value with highest N part, so in late flower it should not be used. Growing in soil if you follow strick ph and watering regime - magnesium should never be an issue too ... Happy Harvesting !!!