Don't forget that last year Charlie exploded from a Cat 2 to a Cat 4 in four hours. I wouldn't expect that to happen with Rita as the conditions are pretty uniform as she crosses the Gulf, but you're going to have to make that decision no later than Thursday evening, and that's probably pushing your luck. It will be the mother of all traffic jams.
There is no evacuation route from San Leon, so the trafic ram will be so bad that only a big cat 4 or 5 would motovate me to attempt it.