Are you stopping to post? Cause that will add time.
I have to say I never have once taught to an empty room. I usually just multiply the # of slides by 2.5 and add 15 minutes per hour that the calculation comes out with. It's usually spot on, give or take 10 mins. If I wrote the ppt.
I didn't write the slides is the problem, and this also lets me go through Brady's stuff to ensure accuracy.