in this DVD - Buon Fresco Foundations - Fresco Plaster
is a chart detailing how thick the wall should be in relation to the fresco you plan to paint on it.
as of how much lime putty you will need per sq. feet of 1" thick (about 5 coats - 3 layers) fresco wall you can see here:
generally it is about 20-24sq feet per 20kg (5gallon) lime putty.
learn fresco at http://FrescoSchool.org
fresco painting video tutorials: http://www.FrescoSchool.com
Fresco School Video Channel
Contemporary Fresco Gazette
commission fresco or mural http://iLAdesigns.com
Contemporary Fresco Painting Resouce Center