For Pin-Masters we're fortunate enough to be able to use the scores from Nationals the day before. We'll typically base our scores on the average of Nationals day for each of the machines.
The best secret I've found for determining par scores when timing is an issue is to simply play a game for 5 minutes, or 3 minutes, or whatever you want the average game time to be. However many points you've earned in that time you want to budget for the game, use that as the par score. Even if it takes you multiple games to play for those X number of minutes, it'll give you a good target.