Heya Mark I use the Seachem stuff as well but don't bother going through all the calculation like you do. I know that you do have to use quite a bit though when you are raising it that much. What I do is mix about 3 tblsp. at a time in some tank water to dissolve and then add it to my sump. Test the next day - see where I am at as far as mg. level goes and just keep repeating the process every day until I am at where I want to be. I have never had to raise it that much but to raise it even 100 points takes me about a week to do. I find that this is a good way to monitor and not go too quickly. Besides with a bunch of live rock in there I am never sure of what my exact water volume is.
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