Was mentioned in another thread , I knew there was site where how to do this was shown.
Found it again in my bookmarked links while looking for something else , so here it is :
http://homepage.ntlworld.com/molyned...ian_plan_1.htm
He mentions a freeware program that does the calcs for baffle rib spacing in his explanation , it's called NEWT, here it is :
http://www.dalekeller.net/ATM/newton...t/newtsoft.htm , a small download. Seems to work OK , I have it on a USB memory stick .
Hope that helps.