John, I just turned 28 a few months ago and I'm shooting for a NFO slot as well. From what my recruiter told me there is somethig like 70 slots still opne for NFO. This is just what my recruiter told me, and I've seen plenty of posts where guys are told different things by recruiters all the time. Plus the new fiscal year starts in Oct so I imagine the navy will have some more slots opening up. From everything that I've been able to gather since starting the application process, as far as NFO's are concerned, ryan hit the nail on the head. They're always needed.
As far as advice for the ASTB. I just took it last week. My scores were ok, 6, 5, and OAR 53. The test it self is not hard. It's the time that gets in the way. The math is weighted pretty high for your FOFAR, and in turn, the mechanical section is weighted higher for the AQR. So those sections are key. The math was my worst section (24 correct), while the mechanical was my best (only missed 2). That's why there is a gap between my AQR and FOFAR scores.
Really the best advice I could give since you are going tomorrow is just work quickly. If you come to a question that you don't recognize how to solve right away, skip it and move on. When you've finished the last question, then go back and figure out the ones you had trouble with. At least you've been able to answer all the questions you actually knew how to solve. Even if you are out of time you should have time after one of the other sections to go back and just fill in bubbles. Remember, wrong answers don't count against you, it's the number of correct answers the determines your score. If you've spent some time studying, the test shouldn't be a problem for you. Like I said, in general I was suprised at how easy some of the questions were, it's just the short amount of time you have that makes if difficult.
Hope this helps and let me know how you did.
Good luck,
Chris