It's best to set the deflectors to a position where they do not deflect the air! If you deflect the air, you will cause a suction on the low pressure side of the wing which will suck chaff down through the sieve. I learned this from Ray Stueckle's book "Combine Settings for Better Harvesting". I found it to be true. In fact Ray suggests you throw out the deflector boards all together. That way they cannot cause a down-draft. You set your wind to get what you consider to be an acceptable sample. If you set it on the low end, you will capture the small kernels and a bit more chaff. Some farmers prefer the low end, as that way they don't loose any crop out the back. The sample may be dirtier, but the dockage is directly proportional to the dirt. The idea here is you don't loose anything of value. Their samples are nothing to be proud of, but they say it pays better. We're only talking a couple of percent difference here. If you have a problem with fusarium mold, you want to set the fan near as high as it will go, blowing out the infected shrivelled kernels and reducing your risk of being badlly downgraded.