Rock....your ph control kit has ph up and ph down? Good for you to find an inexpensive tds meter.....you'll be VERY happy later.
If that cup has vermeculite and perlite......when you go to move the seedling won't all that stuff just fall apart?
I would do this expirement with your tap water......take some in a bowl or something and adjust the ph to 5.6......leave overnight.....check a few times......see if the ph stays were you put it or if not.....how stable DOES it stay when you set the ph? This will be your deciding factor on your tap water.....does the ph stay where you want?
If so, you are good to go with it and won't need any CalMag. The only real way to know is to test it.
