jparks,
I worked on a similar problem in a future life. Never got the program to work right. The problem is not trivial by any means. The only way to capture how the pressure/temperature in the tank is changing is to perform a transient analysis as suggested by butelja.
You might want to look at a few references to see how others have done it: (1) "Sizing an Emergency Venting System for a Cryogenic Dewar", Liu,C.K. et.al., Advances in Cryogenic Engineering, Volume 31, 1986, pp. 973 - 981, (2) "Blow-down analysis of Helium from a Cryogenic Dewar", Khan, H.J., et.al., Cryogenics, Vol. 32, 1992, pp. 388 - 398, (3) Introduction to Unsteady Thermofluid Mechanics, Moody, F.J., John Wiley & Sons, 1990, especially Chapters 2 and 6.
I hope this helps. Good luck.