The FITC channel as well as the PE channel of the 488 laser are typically good channels for AF. There are also channels in the 450-550nm emission range off the violet or UV channels that will also be good to check AF. This obviously depends on the AF spectral signature of your cells, so trying out a few channels where you see an obvious diagonal shape in the unstained population(s) will do the trick. But definitely not random :)

You want to find the minimum amount of antibody that will give you maximum separation. The less antibody you use, the more money you save and you also reduce the likelihood of any unspecific staining. But you need enough to maximize the separation between negative and positive populations for that specific marker you're measuring. Sometimes, however, if you already have a really good separation, and you are not looking at differences in levels of expression, you can reduce the amount of antibody to minimize any potential spillover signal of that conjugated fluor in other channels. You still end end up a good separation, but you reduced the amount of signal that can potentially spillover into other channels.