HI Andy,
You have forgotten to take into account the BW for signalling to make the TRX operate. So the BW for a TRX is 8 * 16k plus 16 k for signalling if TG concentration is used concentration factor=4, give the command RXMOP ont TG, and see the parameter CONFACT if is set to 4. therefore the BW for one TRX is 128+16 equal 144 kb/s without EDGE.
Can you please let me know how you have calculated the BW for a EDGE TRX.
Best regards