Handy Narration Estimator
I found this handy tool awhile ago, lost it, then found it again. I think you could easily do the same thing with word count and a quick mental calculation. But for those that are lazy (meaning me) tools like this are really handy.
I found that my narrator read just a little slower than the medium speed detent and a little faster than the next detent down. For my current course, I'm estimating the experience share for each screen by dropping my script estimates into a spreadsheet. For my tallies I multiplied by the factor of known difference between my narrator's average and the estimator (which was about 1.2).