There is the highest legal rate and the highest recommended rate. With a caustic chemical I’m not sure the best tactic is to run with the highest rate the law allows. I usually use the highest legal rate with insecticides, but that is generally the highest recommended rate as well. As this label shows, recommendations vary with the issue addressed.
Cornell recommends rates lower than those suggested here. I don’t think this is a question that is answered with a single number, but I have never relied on Captan and am just basing this on the sources I happen to have read.
I may be a bit cautious about Captan because the first large orchard I ever managed was sprayed independently of me and it was dosed with a heavy coating of Captan about every 2 weeks throughout the season- every tree in the orchard.
I don’t know what his rates were but the estate was managed with intense fear of failure and independent contractors were fired at the whim of an estate manager from hell.
At any rate, there was no brown rot or scab, but apple trees were ridden with mites and many plums had shot holes in the leaves. Nothing looked real healthy. I always suspected the constant very visible coating of Captan was at least partially the problem, for whatever that’s worth.
So when I use Captan, I use it at rates where you don’t see it on the trees or fruit. For brown rot I rely on Indar and Pristine. For home growers I recommend Monterey Fungus Fighter.
These compounds have less impact on the ecology of the trees, if only because they don’t leave a white dust that mites thrive in. They also don’t wash off in the rain and have kick back, which makes them far more reliable for someone who can’t constantly be on call to reapply fungicide after every heavy rain.