ErlangB – GSM – shared TRX (in & out)

Hi

I am out of idea right now and requesting any open idea

ok, based on ErlangB formula I could perform planning & optimization on my network

Let say I have (based on observation record)
TRX(N) = 2 (14 channels)
Call drop = 10% (Busy Hour)
Erlang = 9.47 (Busy Hour)

now, I want to reduce the drop to 2%, and keep Erlang constant, the result N around 16.

The problem comes in N, coz the 14 channels are shared for incoming and outgoing(that is why Erlang 9.47 instead of 11.5). Hence, the N value is not really 14,

Anybody could help me out on how to make “more” practical calculation?