You can use this web based calculator.
http://appserver.ittind.com/software/HSapps/steamcalculators.htm
For orifice sizes use these for argument sake:
Use 1/16", 1/8" or 1/4" for FT, IB and TH type traps
Use 1/4" or 3/16" for TD type traps.
This should put you in close range to actual...