I found 2 references of interest for Japanese scroll mounting - Chris A. Paschke's article, and the blog by Daniel M. Burkus. The discussion by Burkus is very interesting, explaining why the scrolls are made the way they are, and why they should be hung crooked. But it is difficult to read. The article by Paschke is also illuminating; after re-reading it several times I believe I have the proportions for my piece as a scroll. As in everything Japanese, the rules are very rigid. I will not be able to follow it to the letter - I'll call it an "adaptation" (like a Hollywood movie adaptation of Hamlet, e.g.). According to both articles, the gross dimension of the lower section is half of the upper section. There are also guidelines for color and pattern for the different parts depending on the historic period, to be absolutely authentic.
I will meditate on this while I start steaming and finishing my Falling Stars.