You don't have configuration in your signature, so we can't tell what application you're looking at, but since you're asking about interactive users, I assume you might be using WorldSoftware.
I agree with the other response that said it's a black art best left to the consultants at IBM, but if it helps any, we are a World shop running on an old model 720 which is rated at 720 CPW total, with an interactive feature limited to 240 CPW. We are licensed for 80 concurrent users, but the average load is 60 -65. The box doesn't exactly fly, but it's adequate.
The CPW numbers on the new i5 systems are pretty impressive by comparison, and I'm looking forward to upgrading.
Hope this helps.