Loc - Thank you!!!

I just started flowering 2? days ago and am at 1200 for 3 containers that are linked via tubing. Another one is only at 900. Not sure why. I want to up the 900ppm plant to 1200, but I'm confused as to how I do that.
Do I mix the gh formula in a container and add just a little here & there till it reaches that number? That's the only thing I can think of.

I'm growing Bubblicious now. I did go to the Bluelab site but didn't see anything regarding the 500 vs. 700 but I am now 99% sure you are correct on the 500. Thank you!