if you look at the current erlang traffic, you’re looking at the traffic currently handled by existing TRX. Meaning that you’ll look at numbers such as 15 erlangs in cell A, and 8 erlang in cell B…
but then… how can you decide to add TRX with these numbers ? This traffic is already carried.
You should look at the RTCH congestion, and the amount of directed retry & traffic HO. These will tell you how much traffic is NOT carried.
If it’s too high, then you should add a TRX (or activate HR).
Anyway, when you do dimensioning, always use the maximum value (or average of 3 max values over one month without taking into account the worst one, etc..)
Keep in mind that your subscribers base will grow up over time, and in one year your traffic might have increased by 30%.
It might be smart to include this margin (30% or less) in your computation.