Yes Mark, you make very points on measuring. You and I have what I consider good understanding of batteries and plenty of experience. Much of what we discuss here is more educational in nature. Things are not always the way the book says, the assumptions being made here is that the OP's batteries are not degraded. Down the road when the batteries are not tip top shape, worst case situation happen not due to the load but by lack of margin in the initial calculations, assumptions and measurements.