Here's what I would do......

you have 500 CalMag.....Add 500 Pro grow....dilute entire thing down to 400 ppm....save the rest for a few days to add later.....it will stay good for a couple days....but not too long after that.

The ratios are very important here.

Ps...if it's easier to go to the nearest 7-11 and get distilled water that will work fine too.