To figure out a good mix for your soil, make up some test bricks. Try various recipes and combinations, making sure you label each one carefully so you can reproduce the best ones.
Make your first couple of test bricks out of the soil from the site or from where you've dug out for the foundation. If you're lucky, you may have a soil that works for cob as it is without adjusting the sand/clay ratio. After the jar test, you'll have some idea what you might want to add for the other test bricks. Make tests adding more and more sand until you've stretched it to the point of too much sand. You'll be able to recognize that point because the mix won't hold together well and will be hard to form into bricks. The sand will slough off when you rub the dried brick. You can take the experiment to the opposite extreme and add too much clay so the brick cracks while drying. You will learn a lot doing these tests. Make three samples of each sand/clay mix, adding different amounts of straw until you get a feel for how much you need.
One easy way to check the clay shrinkage in your test bricks is to bury in a stick the same length as the brick. The cob might shrink but the stick won't. You can write the recipe for that brick on the stick with a permanent felt pen.
When the bricks have dried completely, have a look at them. Eliminate the ones with the biggest cracks because they have too much clay in them. Does the sand come off when you rub the bricks? If so, those have too much sand. Try breaking the others. You can drop them from one or two feet up to test their strength. The ones that are hardest to break are the best recipes. Leave them out in the weather or simulate some rain and observe which ones are the most resilient. Analyze what amounts of straw made the strongest bricks. Now you have a good basic recipe. Lots of variations will work just fine, so don't be too much of a perfectionist about all this. You may want to stretch the recipe so you can use as big a proportion as possible of the soil you have on-site.
If you want to reassure yourself and do a more extensive test, build a garden wall, an outdoor sculpture, or a bench, and observe how hard it dries and how well it weathers. Remember, your walls will be protected from the rain by a roof.