Rurumo had some good advice. As for me, I most often just use cheap bargain fert for my young seedlings/cuttings at really low EC (in Pro-Mix) so I can't offer you much advice there. When they get older, I usually use Mega Crop at around the typical 2:3 ratio, with 2 parts calcium nitrate to 3 parts "Mega Crop A", maxing out at about 1.4 EC. It's already fairly high in magnesium sulfate, so if you do want to add the Epsom, go lightly at first. I have done it at .5 gram/gallon to 1 gram/gallon. I don't think it's needed though, tbh... I'm just a fan of super dark green leaves.
You mentioned that your water ppm is 300. Do you know what percentage of that is calcium? Because if that's mostly calcium, you might find good results with just using the Mega Crop A all by itself, without adding any calcium nitrate at all. The amount of nitrogen in the Mega Crop A is adequate for most situations (in Pro-Mix, with average light conditions), so the addition of calcium nitrate is mostly for the presence of the calcium. If you start getting spots or limp leaves, you could always add it back in. This advice might not apply to coco, DWC, etc...
If all of this is too much to bother, just try using it the same way that you would use your Jack's.