The amount of rub is always determined by the size of the slab/brisket/butt/bird ... my advice is to use a binder and then liberally apply and "massage" until you like what you see. Not scientific, but it does the trick.
Yellow mustard is a great choice because it doesn't the final product ... oddly enough, the mustard cooks away leaving no color, aroma, or flavor.
Oil is often used as a binding agent as many spices are oil-soluble. Olive oil, canola oil, or even clarified butter are all good choices though each brings a unique flavor component to the party. I've successfully used bacon grease as the binding agent and I've heard of others using rendered beef tallow.
I've heard of some using molasses as their binding agent, as well as honey. A wet rub can forgo the binding agent (using Worcestershire, their BBQ sauce, or things like balsamic vinegar, etc.) to help make a paste out of your dry rub that adheres to the meat product. A savory or sweet wet rub might help with the flavor you're seeking.
My advice is that yellow mustard is the Chevy Cavalier of the bbq world ... it doesn't do much but it does the job. Beyond this, try to find one that compliments your desired flavor profile of the final product.