It would seem you are right on track formula wise. The key, of course, is to layer everything while it is 'green' to adhere the layers. Otherwise you will have to use a little 'concrete bonder' solution to 'glue' the layers together. This works real well unless there is a lot of moisture - a highly undesirable situation I'm sure.

Fr Dunstan of the Mission Abbey here in British Columbia is finally finishing his fresco.
<http://sck.ca/The%20Monks_files/the%20monks.htm>
He is very experienced - you might contact him as well.

My friend went to see him recently. He was working with a young assistant who was maybe a little bit missing. When my friend started talking about Mexican fresco, the previously quiet assistant angrily replied 'fresco is a religious art - for religion only'. Needless to say my friend was shocked, and I'm sure Fr Dunstan was embarassed too.