I'm not sure what you mean by how far away did the McStay defendant's phone ping. Lol. The search warrant says his phone pinged the tower closest to the gravesite. Of course, he will probably argue that he was visiting his sister a few miles away or driving past on the 15 fwy but coupled with all of the other circumstantial evidence, I don't think anyone will buy it.
It was comforting to read though that cellphone pings are in fact considered when attemping to establish or dismiss an alibi. The search warrants in the Capobianco case have not been released I think for obvious reasons but I'm sure they detail an indepth investigation. Steven C has not provided a public alibi for the late evening. And like the McStay case, the defendant has tripped himself up by giving a public interview.
Some tips for getting away with a crime: leave your cellphone at home and do not under any circumstance talk to the media.