You should shoot for as high as you can score on every PRT/PFA. You need at least a "Good Low" for the mid-PFA, but the final PFA was generally everyone's best as candios. It's ran more like a standard Navy PRT/PFA -- in my experience they weren't really sharking us like they did on our first three PRTs. Make sure your form is good: a lot of people roll on the IST because they haven't been using good form. I noticed during my time aboard OCS fleeters are not immune to this because in the fleet they're not sticklers on form -- any vertical movement counts as a pushup; this is not so at OCS. A good bunch rolled because of bad form across the classes who came through during my time.
My worst PRT/PFA was the IST because you're so worn down the first few weeks there -- I think it was an "Excellent Low." However, I got "Outstanding Low" on my last. It depends on the person, but rolling on the out-PFA wasn't even a concern. You run so many at OCS, you'll be good by that point. Coming into OCS, make sure you're doing your best with good form. When you get to the fleet, officers have more authority and responsibility-- obviously-- and it frankly looks bad if you can barely pass a PFA/PRT, so do well. The test isn't really hard