Hi Brian,
Another good place to check out various scarf joint design options is in pages 263 - 271 of English Historic Carpentry by Cecil A Hewett (ISBN 0-85033-354-7) published by Philimore.
I think that this book is now avaiable in a paperback version in the USA. Unfortunately Cecil only illustrates the joints and gives no dimensions but as per the advice above you can easily figure out proportionate dimensions by applying Jim's simple rule of thumb.
Regards
Ken Hume
http://www.kfhume.freeserve.co.uk
