Milt,
I derived it years ago from the equations in
Donald McClean's "Automatic Flight Control Systems". (Prof. McClean was one of my flight mechanics teachers when I was a student, although I think he's been retired almost a decade now).
If it's important PM me and I'll dig my copy out and copy/post the relevant pages for you (since I think it's been out of print for about a dozen years).
G
N.B. Apologies for accidentally editing your post - finger trouble on my part. Hopefully I've put it back the way you wrote it.