Cob Calculators?

Hi-
I've been working on a small (roughly 300 sq feet) earthbag house in the Upper Peninsula of Michigan for several years now. The "building" season up there is pretty short so this June I will just be getting to the cobbing part. I keep it covered in construction grade plastic when I'm not doing work on it.
Seeing as I live 7 hours from building site, and the remote area its being built in does not have much to offer for materials- I have to haul most of them up. I want to get a rough idea of how much bagged clay I'll need to bring up, as well as a rough count of straw bales and yards of sand Ill need delivered. I know there's no one for all recipe for cob, but does any one have any sort of rough calculations for the amounts of materials vs. sq footage being covered?

Really rough estimating:

(Wall perimeter x wall height x 1.5' (average thickness of cob) ) = V (volume of cob)

V / 27 = # of yards of cob

15-20% of this is clay and 80-85% is sand

Have you used your local dirt for this project or are you only getting bagged clay and sand from off site?

Straw really doesn't take up very much room in cob so I wouldn't count that toward your total volume. My 350 sq foot cob house used 2 half ton bales of straw (256 square feet of straw)

Are you asking about earth plaster over your already-built earth bags? Or are you going to build some portions of cob?

I'm talking about an earthen plaster over the earthbags..

There's actually only about an inch or two of top soil on my ten acres, once you get past that its all sand! I had to have two dump trucks deliver my fill for the bags (road base gravel)
I can't find any clay sources in the Upper Pen.  so I will be relying on bagged powder clay from a source near my home. Just have to figure out how to get such a massive amount moved up north efficiently.

Heres a few pictures of the project.

I was going to ask what materials you have on hand at the property but those pictures answered my question.

Your subsoil looks like clay. Why can you not use that?

All the jar tests that have been done have failed miserably.
Hopefully snow permitting I'll be able to get up there a few weeks before the cobbing and try a few other areas of the property. That would be a dream come true if its usable!