Just to back up a little, I may be wrong but I thought Rostmalz was some type of roasted malt, not crystal. I don't have JZ's book, so I know nothing about his recipe for a bock. I'd appreciate some clarification on the topic if anyone has it.
Tom: I used the Wyermann Dark Munich blended with their regular Munich the last time I made a bock. It's been a couple of years and I don't recall the lovibond. I don't even use Wyermann products anylonger. I use Best Malz for almost all of my German grains, but I have not used the Best Dark Munich (though I use their regular Munich all the timee and it is awesome.). If I remember correctly I used about 50/50 blend of dark and reg. Wyermann munich the last time I brewed a bock. I think I used a little caramunich as well. Maybe a touch of carafa special II. IMO that's all you need. And some hops, of course.