<font color=blue>I do my best to keep the rad full and clean. I do see the odd drip coming out of the overflow every now and then</font color=blue>
I'll assume your tractor has no "coolant recovery jug" since my B7100 did not. Therefore, if the radiator is full, as the coolant warms and expands, it pushes some out the overflow tube, then as it cools and contracts, it will be down a little bit if you check it cold. In other words, there's no way you can keep it full to the top, it isn't necessary for it to be full to the top, and it sounds as if your situation is normal.
Personally, I was concerned about the lack of a water pump, lack of a temperature gauge, and lack of a coolant recovery jug when I had a B7100, and I considered adding a coolant recovery jug so it would stay full on the theory that not letting any air into the radiator would reduce the chances of rust forming. However, a Kubota factory service rep told me that IF it started to overheat, I would hear it "whistling" and it never did that. I didn't think to ask him at the time, but wondered later if adding a coolant recovery jug might prevent that audible warning in the event of overheating, so I never did any modifications at all to it and never had a problem.