Kapem,
If you reference page 45 and 46 in the Ponds, Planning, Designs, Construction it discusses recommended top widths based on the height of your dam not necessarily acre feet of water in the pond. Also has recommendations for dam slopes based on what type of soil is being used. If you know the dam height, top width, and slope, the base should be relatively easy to calculate. There is a note in the pages that recommends minimum of 16ft top width if you are using as a roadway or driveway. I think this answers your question. I would still recommend reading "Just Add Water." The USDA guide is a great technical document especially when having to deal with local/federal permitting. Mike's book is 40+ years of experience that the USDA guide will never be able to replicate.