There's really no way anybody outside of Harcourt to give you an accurate answer. The scale used to calculate your scaled score changes for every administration and form of the test. Easier PCATs will have a narrower scale where you can miss fewer questions than on a more difficult PCAT to get an 85. For example, and this is just an example with hypothetical numbers, you can miss five questions to result in a 85th percentile on one form of the exam but you could miss eight questions and still get an 85 on a more difficult PCAT. Likewise, the scale is different across sections of the same PCAT.
If I had to guess, I would guess based on my experiences that you can miss 20% of the questions to get an 85th percentile equivalent. However, that is a very, very, rough estimate that you shouldn't take to heart. I recommend that you at least take the Harcourt practice exam to get some idea of how your performance on a PCAT-like exam could influence your future note.
P.S. For you and this board, please keep any scoring and score interpretation questions in that "Yay!" thread on the first page. It'll reduce board clutter while adding to a thread that could be used by future PCAT takers.🙂