OK I am confused. The reference link does not take into account the BB size, 68/72 or the rear spacing and cog cluster dimensions. Isn't the chain line determined by those references too? I assumed the datum is the rear configuration as long as the inner chain ring does not hit the chain stay. Why Sheldon starts with the crank is a bit confusing to me too. I would determine the measuremetn from the rear and then match the front as close as possible and then use the rear axle cone placement for fine tuning.

http://sheldonbrown.com/chainline.html

I am not a mechanic just a dumb mechanical engineer. How do you mechanics measure, rear first then cranK?