I didnt start using cal mag in my first grow until it was almost time to flush for harvest, the only thing I saw that I thought was weird was hollow stems, but that could have also been just from using LED's. Honestly, I havent really had a cal mag deficiency, so I've never really needed it. I guess what I'm trying to say is, I would keep it around just in case... it's better to have it and not need it than need it and not have it. But Fox farms makes their own cal mag, I imagine any would work fine but if you're just starting out I'd probably run with that. Are you running your plant in coco coir or regular soil? If regular soil, I'd go with Fox Farm Happy frog, it's not as hot as Ocean Forest and is better suited for autos. Wouldn't have to use your nutes for the first 2 1/2 - 3 weeks or so.