There is some ofrumlae available at
http://www-staff.lboro.ac.uk/~elvpc/progs/expch.html

I suggest the following workplan

1. use the sums to get you in the ballpark
2. make a set, see how it perfoms on the dyno, measure the gas temperature in the three sections
3. make another set using the revised temperatures and knowledge of power delivery to lengthen/shorten teh sections
4. go back to 2 and repeat adinfinitum, or at least until you are satisfied with the performance

Steps two onwards get you in front of the goal posts - how close depends on how much patience you have. It is clear now why Andy wants money for his hard earned drawings - it takes time and effort to make a few sets and associated dyno time (with rejetting and ignition optimisation for each set) to get something he was happy with.

There are also software available which does the calcuations if you look hard enough on the internet - try typing expansion chamber design software into google, like this.

A better decription of expansion chamber design, tied to port time area, is available in the book by John Wood at http://www.lortim.demon.co.uk - if he can get a 200cc villiers engine from 8bhp to 40+bhp, he must know his onions.
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NSRless again...