Alright lads don't quote me on this but I think the reason they don't put to much cal mag in the base nutes is because to much cal can bind up other nutes.
On the canna nutes thing in coco all you really need is cal/mag to go with ya A n B, the A n B is a complete feed so you could easily get away with with spending just £20 all the other additives are down to preference and defo not essential, I'm 5wks into flower and have got away with using just the above nutes.
IMO the vast majority of the stuff people will tell you that you have to have is down to marketing and forum bullshit, many people get great yields using nothing more than a solid base nute.