Manny, I had the exact same experience! I expect you will too Ray, once you are a little farther into it. By the time I was finishing my bench I was digging drainage ditches and using whatever came out for the bench mass. I started out worrying myself silly over which clay and ratios and everything you write about there Ray.
What Manny and I both figured out Ray is that most of our soils are already a pretty good mix. Go to a spot near by your work area where you can dig. Clear away whatever is on top, grass or forest duff or whatever so you can see the dirt. If the dirt looks like you want to plant some veggies there, dig deeper. Once you get to the subsoil, dig a pretty good pile of that onto your cobbing tarp and add water and start stomping. I bet you'll find that you have a pretty good mix naturally without having to do too much combining of sand/clay. You can work a lot faster this way and quickly make large batches. Trying to mix clean clay with clean sand is a frustrating process. Like Manny said, once you get it wet enough you can then tie it all together again and dry it a bit by adding in the straw. I don't really pull off a clump as much as make a ball, or a "cob", and toss it back and forth between my hands to bind it together as I head for placing it.
A little clay/sand bedding is the perfect way to fit a riser, in my opinion. Glad to hear you got a good burn going, amazing what a difference that insulated riser makes isn't it?