I'd say a percentage like yours in the mid-80s is good.
However, what's exponentially more important is what you can learn from the questions you got wrong. The great part about the Self-Assessments is that they can really help diagnose your weak areas and allow you to tailor your studying to hit your weak spots. And I'm not just talking about content areas either. Make an effort to fully understand why you got the questions wrong, and then formulate a study plan to fix it.
For example, if you're making too many "dumb" mistakes, consider slowing down during a FL and concentrating a little more during each question. Perhaps some questions are wrong due to missing details in the passage. Or maybe you're missing key words in the question stem. Or, of course, maybe it's just content deficiency.
The point is, scoring well on assessments can be a confidence boost, but that isn't what they're there for, IMO. The key to improving on the MCAT is turning weaknesses into strengths. The first step to making that transition, though, is knowing what your weaknesses are -- all of them. Diagnosing them will make your studying more efficient, and, hopefully, your MCAT score improve.