I don't think course par enters into the PDGA rating calculation at all. It's based on how other players scored, relative to their player ratings.
"The scores thrown by propagators each round are used to calculate the SSA rating for a specific course layout. A propagator is a current member whose rating is over 799 and is based on at least 8 rounds. As long as there are 5 propagators playing a course layout, ratings can be calculated. The average rating of all propagators will equal the average rating they get for the round – always. If the same course layout is used more than one round, the scores from multiple rounds will be used to determine an overall SSA, as long as the individual round SSAs come out close to each other, so everyone gets the same rating for the same score on the same course. If the SSAs are significantly different, likely due to varying wind conditions, the round ratings will be calculated separately. If a propagator shoots more than 60 points below their rating, their score will not be used in the SSA calculations. "http://www.pdga.com/faq/