Tobin is correct - you want to get yourself a Mg test kit so that you know what that reading is. While Ca & Alk are important, Mg is the 3rd piece of the puzzle and if it isn't where it should be, you will have a devil of a time keeping your alk up.
Please keep in mind that if you aren't testing for it - you shouldn't be dosing it
Sure fire way to screw up your tank
Kinda wondering why your Ca is at 300..that is definitely on the low side, especially considering you are dosing.
What kind of salt mix are you using?
How much Alk & Ca were/are you dosing? Did you base the amount you are dosing on any particular thing?
My suggestion at this point would be to not dose anything at all until you figure out what your baseline is in your tank as you may not know just how much Ca & Alk your tank is using up (which is how you ulimately figure out how much you need to dose, but I can't explain that at the moment.. may have something to do with the fact that I have been out partying tonight ...
)
The best thing you can do for your tank at any point is a water change - and that would be my suggestion for now
Then leave it alone for a few days and take more readings (including Mg!) and post them here.
One other thing - when you mix up your new batch of SW - test that too. Can be a real eye-opener to see what you are truly adding to your tank. Would like to see those readings as well if possible.