Cannabis likes its environment slightly acidic, that is the ph should be about 6.5. By "runoff", I mean the water that is coming out of the drainage holes on the bottom of your pot after you water your plants. Test the ph of the runoff and you will get a true reading of the ph of your soil. Flushing is done by, as someone stated above, watering with roughly 3 times the volume of your container. I know it seems like a lot, but the excess simply drains out the bottom, so you're really only maxing out the soils holding capacity. This will help clear out any unused exessive nutrients and prevent the dreaded "nute lock." Sucks you went with crappy MG stuff for your grow, next time grow organic, and you and your plants will be happier.
Oh one more thing I almost forgot, DO NOT WATER YOUR PLANTS WITH WATER STRAIGHT FROM THE TAP. Chlorine, man, CHLORINE. If you are going to use tap water on your plants let it sit out over night to let any chlorine evaporate. Then I guess it should be fine.