The numbers are going to depend on your guide camera (sensor size, pixel size) and focal length of your guide scope, or if you're using an OAG.
In my experience you need a very good polar alignment and good balance otherwise PHD will be all over the place.
You can use PHD to drift align though which makes it a fair bit easier, I use the technique described here:
http://njstargazer.org/PolarAlignment.asp