I add my amendments first, like silica, cal-mag and mineral matrix. Then, I add the Jacks mix in equal amounts until I get the EC I'm looking for. Trying to determine exactly how much to mix up ahead of time has always seemed like more of a pain in the ass than it was worth, so I don't do it.
So for example, if I'm mixing a batch of Jacks, I might mix up a 50 gallon batch like this;
5 Gallon Bucket A:
3 Jacks x 50gal = 150mg
1 Epsom x 50gal = 50mg
5 Gallon Bucket B:
2 Calcium nitrate x 50gal = 100 mg
Fill both buckets full of water and mix thoroughly. Add equal amounts of each bucket to reservoir until you have achieved your target EC.