Take the weight for 100 gallons and divide by 4It is really not that complicated. If you go to the Jacks321 website and look up their feeding schedules they have the ratios and weights right their. They are using 1/100 ratio for injectors. If you want to make stock you can do it based off the ratios. I would not mix the stocks but have 3 separate tanks. Can you mix jacks with Epsom salt probably but that can change how long the stock is good for as well.
If you want to make say a quart of each. Take the weights for the gallon divide by 4 now you have 1 quart or 32oz of each stock. If you want to make 5 gallons of mixed nutrients you can use their formula to figure out the correct amounts. 1 to 100. One gallon is 128 OZ x 5 gallons = 640 oz. So 6 .4 oz of each stock gets you to Jacks at full strength. Basically 1.28 oz of each stock is 1/100.