There are a couple of factors involved in trying to determine the propper PPM range.
1. Different types of strains can handle more PPM than others.
2. The kind of nutes you are using will make a difference in how high of a ppm you can use. My experience is that the more chemical your nutes are(Opposed to organic) The lower the PPM your plants will be able to handle.
3. The kind of water you are using will make a difference in the PPM
To find the maximum PPM you plants can handle you should add nutes every couple of days to raise the PPM.
When you you see the tips of your leaves to burn, You know that you are at your mamimum PPM for that type of strain.
Then just add some water to your nutes to bring the PPM down a little to keep them from burning.
BTW I run my PPM around 900 until the 3rd week of flowering, Then I raise it to 1100.
Aerobuds 