the rust rating is determined by how much rust covers the leaves?
Yes.
One can modify that.
A cultivar that has leaves 20% covered with rust in June should have a different rust resistance rating than one that does not have its leaves 20% covered with rust until November. To make certain those two cultivars receive different rust ratings one would rate them both at the same time (say in June).
More scientifically one would examine the amount of rust on the leaves of the cultivars many times during the year and then graph the results. From the graph one could see when rust first appears on different cultivars, the speed at which rust develops on different cultivars and the final amount of rust that develops. It is possible that all three measures are different between cultivars. On the other hand it may be that the measures are related.
