a few questions:
why do you want to retake?
what is your new target score and how did you decide on that?
Have you analyzed your previous full-lengths in detail for mistakes and strategy problems?
How were your scores on those? Were they congruent with your actual score?
What factors will help you to do better this time than last (i.e. have you figured out what you could have done differently last time)?
Do you have any weak content areas, and how are you going to fix those (essential if you are going to gamble on increasing from a 30)?
Personally, I had done 11 full-length tests from a variety of sources (AAMC, EK, Kaplan), with steadily increasing scores (increasing one point at a time, or holding steady overall). I knew that even on my worst day, being tested on my weakest areas, I would almost certainly get my target score or higher. My actual MCAT score was right in line with my last 3 tests.