Hi! The reason Jeff posted the book and the calculator is because the ratio of clay to sand to straw depends on how much is present in the soil you are using. The Hand-sculpted House does contain a 3:1 ratio for sand and clay, but, if I remember correctly, it is only one of 3 or 4 ratios that have been used all depending on the makeup of the soil, what the climate is like, and the purpose of the cob being made.
I'm afraid cob building is not a spur of the moment project; it requires some forethought, and calculation. If you read The Hand-Sculpted House you will know why. I found it a fun and interesting book to read.
Good Luck!
BTW, if you do by some chance find someone who gives you a ratio for straw, BEWARE! That 3:1 ratio may not be what you need and may result in your shed falling in on itself.
