Okay, if I am understanding you correctly, you were doing your TDS/EC reading AFTER adding your pH adjustment- right?
Here's a better order in which to do this:
Pour your water and measure the EC right outta the tap.
If it is significantly below 300 parts per million, add CalMag Plus up to 300ppm.
This gives you fake 'tap water', lol.
Then add your nutes to the desired level.
Then last, adjust the pH. Your pH up/down does in fact add ionic strength the the water, so check it last.
Good luck1!