OK, I've been testing alk in my fairly new tank (2.5 months) for about 3 weeks. The level has always hovered around 13.8 dKH / 4.9 meq/L. Measured using the Salifert test kit. I'm doing 15% weekly water changes, and using Instant Ocean salt mix.

So that is on the high side, I know. How much should I worry, and if it's necessary, what can I do to bring it down?

Not sure what you can do if your salt mix is causing your high alk, maybe try a different salt. Or maybe your test kit is off, take it to your LFS and have them test it.

Dose with calcium, as I understand it calcium naturally lowers Alk and vice versa.

I've heard that some batches of IO salt mix up with fairly high alk, and most of the time are a bit low on the calcium readings, if you mix your own, you might try stirring up the salt in the container it comes in, making sure its thoroughly mixed, that's a good idea on any salt mix you may get, as some settling/separation may occur while it sits in a warehouse/store/shipping trucks.

I'm actually getting strong results for calc as well... generally 440-540, depending on how close I am to a water change. So perhaps I just need to try a different salt. I only have about 10 gallons worth of my current salt left, so I'll just try a different brand next.

My Alk was also through the roof when I was using IO (13-14) while having low pH (7.8). I switched a couple of weeks back to DD H2Ocean Pro+ and the parameters stabilized. You could try a different salt and see how that works out.