http://www.advancedaquarist.com/2002/11/chemistryGood article above... perhaps much is obvious, and I'm no expert, but some random thoughts...
Low Mag prevents uptake of Ca/Alk (I had 800 mag once
)... if it's not low Mag, and both are low, one must increase both concurrently to get both to increase. If either is proportionately low/high, one can add the missing element until balance is achieved. I assume you’d have to add an ounce at a time alternatively to keep the levels moving up in a balanced way?
13.6 does seem a little high... My regular IO has been mixing up at 11 or so (seems higher than it used to).
I think IO/Reef Crystals has stepped it up so people can use water changes as the only method of supplementation.
If the Calcium is balanced (4x Alk), don't dose/Kalkwasser/Ca reactor the days you do a water change (4x Ca to Alk, so if alk is 11, Ca 440). For example, if your new water is 13 and your current is 9, and you use .4 DKH/day, change 10% of the water will raise you DKH by the .4 you would have dosed. If Alk is high and Ca isn't, add the water and step up the Ca as it's consumed. 1.4 DKH is the max/day advisable in the Bulk Reef Supply calculators. They are great tools for dosing calculations.
http://www.bulkreefsupply.com/reef-calculator