eterry, I find all of your measurements in your first paragraph to be right on! That is exactly what my templates measure when very accurately measured and taped together. My pieces are not sewn together as yet, but if I am bigger than 17" unfinished when sewn, I know my seam allowances were too small. If I am under, then I've made my seam allowances too big.
After reading some of the problems that ladies were having, I took extra special care in putting the templates together, and told my DH, "If I get a bowl or a mophed unit in any way, at least I will know it is my sewing, not the templates!" I think the key to the whole accuracy of the templates, is to measure ALL the way across the grid! I fear some have only measured a 2' square or a 3" square.
Dawn
In beautiful Northwest Montana