That is really unfortunate, Shadow. It is frustrating when you are really close.

As for how many questions/ scoring this is what I have deduced from my reading of the systems and some statistics studies for psych and my eventual stat exam:
The reason the scales are 20-80 with 50 being the passing median is that 99.7% of all scores fall within 3 standard diviations above and below the mean in a normal distribution. So basically they make the passing mark based on the average score of C students 50 and then each standard diviation is worth 10 scaled points. That means that if you scored exactly 1 standard diviation above the mean you would get a 60 and 1 SD below is a 40. So if the average passing grade is 55% and the SD is 12% you would need to score a 67% raw score to get a 60 on the scaled score. In this example, if it was a 100 question test, every 1.2 questions right earns 1 point so to bring your 48 up to a 50 would take 3 questions. Of course to do these calculations you need to know what raw score equals a 50 and what the standard diviation is which I don't know if they post those or not but basically that is how they score. You seem really close so don't be discouraged.

FYI, I calculated that getting a 70-75% raw score on any DSST almost garuntees you at least a 60 which is probably an A at Excelsior no matter what the exam so if you know the answer to half the questions and can narrow down anything you aren't sure of to 1 of 2 answers giving yourself a 50/50 shot of guessing correctly you should always score high enough to not only pass but get an A.