John is the lone SDET on the developer solutions team. He's fairly outnumbered by his developer and PM counterparts so, he spends a good amount of his time working on ways to automate and increase the amount of testing provided by unit tests and the other members of the team. Oh, and now he's blogging to.
My own experience suggests that “average” unit tests hit no more than 50% of the code. However, it is fairly easy to improve that to 65% - 75% code coverage by testing requirements using a “black box” approach. Getting to 85% or above often requires that additional tests be designed by looking at the code and figuring out how to make the code execute an untested path (i.e. “white box” testing).