What does it take to get to the B-/B range in prestige? With a NT bid last season for St Johns I went from C- to C+, then went 19-9 with an RPI of 95 this year and missed the PT. Will my prestige drop to a C because I didn't make the postseason at all?

Yes, it'll drop at least one slot, if not two.  You may be back to C-.   In Naismith DIII, I went from A+ to B+ after I missed the NT

You'll be at a C or C+.  C- is the lowest you'll go at d3 and anytime you win 19 games you'll be better than C- for sure.  Since you had a top 100 RPI I think it's possible you'll stay at C+.  In my first season in Knight with my d3 team I went 22-5 with no postseason and an RPI of 101 and I kept a C+ prestige with no postseason the season before.

So you can't increase prestige without a postseason appearance?

I'm not sure how it works in D3, but you can increase your prestige w/o a postseason appearance in D1 if you've starting in the D/D- range.   At that point, any significant improvement will bump you up 1/3 of a grade.

If your conference went crazy and won a lot I have seen teams who do not make the postseason increase their prestige.

At low enough prestige levels, you will increase (even if it does not show up in the grade letter, like low C- to high C- almost a C).  It is possible to increase from a D- to a D with 14 -16 wins, it is possible to increase even in the C- range to a C with 19 or 20 wins and I have seen this done at all levels in the game.  I am not sure who figured it out first but the first one I saw that was activley using it was furry.  You load your schedule for a year or two with easy teams, get your wins, get better recruits and you are on your way.  Hope that helps you.