ERP Concurrent User Estimator

I found a very interesting spreadsheet called ERP Software Concurrent User Estimator from TGI on this link.

The formula used by them is:

No. of Concurrent Users = (0.3 x Low Activity Users) + (0.6 x Medium Activity Users) + High Activity Users

This is a good estimator which can be used to come up with number of concurrent users, which is needed for sizing an environment.