Dimensioning based on footfall

Hi All

I understand the concepts of traffic engineering, erlangs and BH etc however I am trying to understand if there any simple factors that can or are used in GSM to translate the number of people who may in or pass through any one given location to number of concurrent users and hence traffic and GSM RF channels for that given area.
As an example I know there are 1000 users in a building
They each have an average holding time of 180 seconds ( 50mE)
If this ws busy hour traffic then a total of 50E would be required= 61 trunks or about 8 RF channels.
However I have come across some calculations that show a requirement for the same data to support 5.6 E in effect derating traffic by a factor of 9.

My question is:

Is this an established GSM number and if so where does it come from?
Is it associated with the normal distribution of traffic over the course of a day and making some guesses!

Any answers or assistance would be welcome.