I think mixing tobacco with weed is more of a European thing, and not very popular in the US.

Everytime I've ever smoked weed mixed with tobacco, I felt kind of sick and nauseous. I really can't stand it. I can smolke tobacco on it's own, but I don't usually inhale it as deep as a hit of weed. With straight weed, I can take a nice deep hit, but with any amount of tabacco, I can't get it deep into my lungs, and if I do, that's when I feel sick. Once someone packed some tobacco into my pipe and smoked it, and after that, my pipe tasted like shit for a week --- seemed like it took forever to smoke that crap out of the pipe.

I almost always use a pipe for all my smoking, so I never run into a problem needing to stretch out a small amount of weed into a joint. Just pack in however much you want to smoke, and don't add anything else. It's the most efficient way.