With a new game I can think of three methods,one is to use your beta testers (established players) to set the baseline. Another to use the system as is until a base is established. A third would be to log results for the startup period and run an offline analysis to baseline initial BKR's.
Going back a bit I also think Ratings should have a half life (decay) to make people work to keep them and to offset the influx (i believe the current algorythm has a positive bias) if not then it should have.