Three options: The easy way is to look up some courses on this website to find those with SSA close to the one you're planning to play. Click on the event that produced the ratings for that course and see how much each throw was worth by looking at the round ratings for people one shot apart.
Another option is to subscribe to the Disc Golf United handicapping service ($19.95/yr or $2.95/mon) which does all of the math for you and tracks your personal numbers. http://www.discgolfunited.com
Another option is to download the Excel template for calculating ratings.
Enter a fake player with 1000 rating and put the actual SSA in as their score. Make sure to enter 18 for the number of holes at the top. Then, enter a second player without a rating who has a score exactly one shot higher or lower than the SSA. awitch to the tab that has the ratings on it and look what the difference is between the ratings of those two players.