I answered this for another kid a few days ago, so I'll repost some of what I posted to him because it seems to fit. = )
Your score should not be a problem, remember, it's not your percentage correct that they look at, it's where you are in percentile compared to others who have taken the test.
It looks like at St Francis they admit about 1/3 of applicants, which isn't too bad. It looks from it's AP scholar numbers, SAT scores, etc that it has good academics, but a very wide range of student ability. Not everyone is going to be a top student, the kind of kid who always get straight As. I'd bet there are lots of average kids in the school. (Which is very common with large or Catholic private schools -- there is always a nice mix of kids.) Your HSPT score should be fine based on how you are doing on the practice tests -- even if you don't score as high on the real test.
I doubt that the HSPT score is the main factor from looking at their website. My son has applied to two local private high schools (we're down in LA) and those schools probably have ISEE scores as the number one factor for admittance, along with stellar grades, but the schools have a very rigorous curriculum, everyone takes and passes the AP exams, high percentage end up at Ivy league schools, that kind of thing. Another school he's applied to isn't like that, it's more well-rounded with a wider variety of kids, very similar to St Francis.
I didn't see an average HSPT score for admitted St. Francis applicants. Maybe it's on their website, but if it's not, that would be another indicator that it isn't quite as important as you're thinking.
Lots of my son's friends recently took the HSPT and they said it wasn't as bad as they thought it would be. My son is taking his in a couple of weeks, he already took the ISEE.
I really feel for you kids and these long entrance exams!
Good luck and try not to worry!