[Cob] lime putty calculations

[Tys Sniffen]

Anyone have an idea about how to figure out how much line putty we'll need
for our exterior plaster? 

Obviously, one mixes it with clay and sand as well, but how much putty do I
need to buy? 

We've come up with a surface area of 1500 sq. feet, and it seems like the
easiest and best way to get the lime surface we want is to buy lime putty.
(also interested in sources for that, if anyone has some) 

We're in Northern California.

Tys