hi peter
this is my proposal of calculation:-
BHCA= A*3600/B
A=C/D
D=C/D
where
A= traffic in erlang
C= traffic in erlang per sub
D= total number of sub.
B= Mean holding time in secs.
Here you must know:-
B,C,BHCA, then you get the A. From A you will get D.
good luck.
rgds
mohsin Ismail