Optimum load is a place which allows the blank to perform to the best of its ability without any noticeable performance drop off. So, it bends sweetly, casts a good long line and feels crisp and overall its happy days.
Thank you for that, which came in while I was writing my previous post.
You partly answer my question, but the extract I've quoted is the closest you come to defining optimum load and, as with Fruin's answer, it relies on terms that aren't exactly scientific and are pretty subjective - "sweet" and "crisp" are the two you use, which makes it sound as though we are discussing apples, not rods! And, with respect, load is surely a force (which must be measurable), not a place!
Is there also a scientific, or mechanical, answer - and a way of measuring optimal load for rods - that doesn't rely on subjective assessments like this? And if so, is it useful to know this?