I don't have a webcam, otherwise I would be more than happy to post a video explaining the derivation. It's all simple first year engineering fluids work, I'm really not sure how Mike missed what he did. Would you be interested in me posting a complete derivation with explanation as a paper copy? Then you'd understand more than PhD Mike.
I'd be interested in seeing that. If you can PDF it, you can post that here. Or better, in its own thread as technical critique of the renowned site posted above. Probably best to post into the Brew Science area here. I'll keep an eye out for that.