Hi!
I don't recommend you to buy CalMag until you know what kind of water are u working with. If you don't have EC metering pen, you can check it on the website of your area (it won't be exact, but you can guess approximately). If it's <450ppm you can buy CalMag and add it to your water till you get that measure of PPM.
About PK. It would be a fantastic idea to boost the flowering around the 4-5th week.