If the rate is $300 per hour and you get 2 that are good. What is the proper tip, and when do you give it to her?

The proper tip is what you feel is appropriate.  It can be as high as 30% or it can be nothing, it is all based upon your feeling about the session.
    The tip can be given to the lady when there is a break in the action or just before she or you (if you visited an incall location) departs.