well, you have to consider the average duration of one call, not the erlang traffic on the whole cell !
To find the value, it is easy : erlang divided by number of TCH assignments (for normal assignments + for incoming handovers), and multiply the result by 3600.
But usually you can consider that the average is about 60 seconds… it is a value that you find frequently worldwide.
And usually, you wait an average of 3 seconds before your call is being answered (or before you answer the incoming call).
That is where the 5% come from. You can decide or not to take it into account, but if you want a realistic simulation, you’d better throw it in..
If the cumulated duration of the calls in a cell = 60 minutes (for example), the cumulated duration of the “waiting”/”ringing” phase is about 3 minutes. But that duration is split over the whole amount of calls that were processed in the cell.