You can't assume that since your alk is high, your calcium is low... especially if you are dosing. Get a test kit. It is a bit of a balancing act, but I often find that I am having trouble keeping everything stable, I need to check my Magnesium.
Also, 13 dKH isn't that high, or well not enough to seriously be concerned about. Especially if you are using Instant Ocean or Reef Crystals salt. The published alkalinity at 1.0265 is 12 dKH although a lot of people end up reporting batches of 14 dKH.
As stated above, your Alkalinity will gradually fall as your corals use it. The rate at which it falls all depends on the demand in your tank. You're demand is probably fairly low, so it may take a few days... even a week before it falls into an "acceptable" range.