I looked through Brawn
in the chapter that talks about McCallums ideals. They don't talk about specific BF% for these measurements. They only say a body in a hard condition, rather than a soft one.
That could mean a range of several % either way, but I would think that 12-13% is not out of the question. I think this is a BF% that most people could maintain with some disipline, but not adhering to a super strict diet.
In any case I do think it is a good idea to have some sort of an idea as to what proportion's you should be aiming for. The better your plan, the better the chance you have of success. At any rate these are ideals, and as they point out, very few people are ideal. Not everybody will achieve these stats. Temper your expectations with some honesty about you somotype, but don't let that stop you from busting your butt to achieve 100% of what you are capable of.
Anibal Lopez. very much an ecto type frame. Still the man achieved great success.
He looks great on this cover shot. Ripped, with great symmetry.