physan dilution?

I have a beginner's question. Several of my plants (an oncidium and an odontoglossum) have developed black spots on the leaves. I gather from what I've read here and elsewhere that Physan spray may be helpful. I have the physan; what dilution should I use? The label says 2 teaspoons per 20 gallons. That's the weakest dilution I've seen. In looking at various sources on the web I see dilutions as high as one tablespoon per gallon, and everything in between. It seems that stronger dilutions are appropriate for use on non-plant surfaces (e.g. greenhouse benches, etc.), but the recommendations for what to use on the plants themselves are still all over the place. What's commonly done by posters on this group? Thanks in advance for your help. Chuck B.