Usually you change portrange for every computer.
EX:
Lets say i have 5 computers behind a NAT router i usually forward mabye 10 ports to every singel one.
forward to ->PC1 portrange->50000-50009
forward to ->PC2 portrange->50010-50019
forward to ->PC3 portrange->50020-50029
forward to ->PC4 portrange->50030-50039
forward to ->PC5 portrange->50040-50049
And then i configure all applications on every pc to uses that dedicated portrange.
EX: all p2p programs listen to those portranges and icq,msn and souch.
I have never run inte problems by doing this, if the range is to narrow then open/forward maby 20 ports.
But if you cant change listening range in the application in question then you get into trouble.
Can you say what application it is? (easier to do any recomendation or find solution like special scripts and souch).