200 gallon mix comes like this;
box 1 - pure salt;
box 2 - mag/calcium/alk
that piece of paper is the mixing guide - there is a pdf of it on the ESV site if you are interested.
I need to get a digital scale so I can mix this stuff perfectly.
You can try splitting everything but i dont think this stuff has shelf life as long as you keep it properly sealed. I only have a 45 gallon tank and will do 5 gallon changes weekly so it should last me 6+ months.