@WeedHound - I'm using a Milwaukee with a TDS factor of 0.5. I'm assuming it measures in TDS, there is no documentation that came with it specifically saying it measures in TDS other than the "TDS Factor of 0.5."

About the 120PPM, that number I read off my local water treatment plants website. Now, to take into consideration I live in an older neighborhood built in the 50's and I'm assuming the plumbing infastructure around here is less than to be desired (thats my hypothesis anyways). So after I got my PPM pen home I I tested the water and it came up at 300PPM. I also tested brita filtered water and it also came up at 300PPM. When I did this test I did not let the water sit for a while at first. I then measured the water I was using for my seedlings and it was at 660PPM with 1/2 strength nutes, and my resv. was at 1030PPM. I then tried to nock down the PPM in my resv. by adding 7 gallons tap water and was only able to get it down to 900PPM due to the resv. being almost full to begin with. The brita filter seems to actually raise the PPM when I measure it right after filling the gallon jug however.

I've been doing a top off at the end of the first week between the two week cycles but the last week I didn't. I also didn't understand this whole PPM thing last week either =p My eye's have definately been opened thanks to your help and this wonderful website and I really really appreciate it. I think now I'm in a situation of trying to find the sweet spot in PPM's for the plants. About the CalMag though, how much do you think I should use in PPM?