Hydroponic PPM and feeding. How do I match magnesium sulphate to potassium for flowering??

Unfortunately you have gotten some bad info. Your plant will require little tweaks on the npk through flowering to keep it happy. 1-1-2 is way too low to build fat buds. You do want your ppm to be as high as possible without burning and too much calmag can cause other nutrients to be locked out. If I knew what nutrients you were using I could probably give you a little more info. Most nutrient manufactures have a pretty well balanced feeding schedule other than them being too strong. Usually want to run 3/4 recommended dose.