Brandon,
Most of my jobs, I am the GC and like Chris I add at least 15-20% and sometimes more. It depends on the sub and the work to be completed. If I have to do all the leg work, and hold the clients hand constantly I will charge 30%.
I have been very fortunate to have clients that trust me and allow me to take care of their projects, allowing them more free time and taking care of the problems when they arise, which will happen every once in awhile.
I will say there are some people that will not pay this and want to bargain with you. It has happened and I have made it very clear that I was not the GC and they were to handle the scheduling, other subs, etc.
Dan