[phenixbb] Designing heavy atom positions on my DNA crystal

Hi everyone, I am trying to grow my DNA crystal with bromide derivatives. The content in one asymmetric unit is a 30 bp DNA duplex. I am considering about how many bromides modified bases should I use to ensurethe strong enough heavy atom signal. Also where to put these bromides? To my understanding, maybe I should put the heavy atoms at the most rigid places, avoiding the clashes and not too close to other heavy atoms. I have few experience on this and hope you could give me some help on the design. Thanks a lot! Sincerely, Xiang