Deviant,
What you need is to use Bernoulli’s Equation. Do a google search on it – there will be lots of hits and info on how to use it. Here is one link:
http://www.grc.nasa.gov/WWW/K-12/airplane/bern.html
With out going through numbers in your example, if your nozzle is releasing to the atmosphere, then you will only have dynamic pressure at this point (rV*V/2, r = density of fluid, V = Velocity). This will be equal to the static pressure in your system, which was explained how to get in the above posts.
Try working through it, if you are still having trouble, give a shout and I’ll show you some more.
Hope this helps. . .
plm