It also varies among exams too. If you go by Kaplan's score chart from their book, 24/40 is a 5. However if you go from the scale on AAMC 6R, then its a 7. My Princeton Review online VR section gave me a 10 for getting 77.5% right though.
Here's the scale fro Kaplan's "New MCAT Practice Tests, 5th Edition" (p128):
Raw Score (Estimated Scaled Score)
23-24 (5)
25-27 (6)
28-29 (7)
30-32 (8)
33-34 (9)
35 (10)
36 (11)
37 (12)
38 (13)
39 (14)
40 (15)
Beats me if this is accurate. I ommitted scores <5 since...well its kinda obvious. Anyway...depending on difficulty, these scores will probably vary by +/- 2 points as well. But having taken the MCAT back in 2003, you should be getting about 70% or more right. There's not a lot of room for error on VR.