It is not compalsary to use the ratio B:A as 1:8,U can go for 1:15 or 1:24 or 1:35....
but the smallest possible best matching combination in the layout point of view is the 1:8 The advantages in using this is the less area,best matching.the arrangement is :
A A A
A B A
A A A
Even u can go for the 1:24 or 1:35 there also u will get the common centroid but the area will be huge compared to the rest of the circuit.
u might get the DOUBT that B:A = 1:3 also the smallest possible common centroid arrangement,but it is not,since the arrangement u will get is:
A A
A B
In the above arrangement to provide the same environment for the all BJTS u need to place again 5 more dummies, which comes to the same arrangement as stated B:A=1:8.
So most of the BGR designs will contain the 1:8 ratioed BJTS
vijay