A lot of people (me included) just mix up IO, then pour in a bit of concentrated calcium chloride (since the calcium is low) and a bit of concentrated magnesium chloride (since the magnesium is also a bit low). I have 2 liter containers of each of these next to my 25 gallon mixing trash can. I just mix up the salt, then dump the right number of capfuls of each Mg, and Ca into the bucket (takes an extra 30 seconds). It's a good idea to wait until the IO is mixed up before adding the calcium as it might be more difficult to get your IO to dissolve. This is part of the reason that IO is a bit low in calcium. Most people don't have corals, and don't care about the calcium level. They just want the salt mix to dissolve quickly. Can you imagine the number of people that would screw up the mixing of salt if IO came with some extra bottles of liquid Ca/Mg that you had to add separately?!?