When adjusting the dollar duration and using one controlling bond instead of buying a bit more of each in the portfolio, do you still need the DDold/DDnew -1 formula?
You can just do DDold-DDnew = DD adjustment. Then you need to buy as much of the controlling bond to = the adjustment.
I doubt this question will come up. But if it does, choose the bond with the highest D as it gives more bang for your buck.