MGBF (maximum gain before feedback) can be calculated. The biggest factors are how loud your source is, how far the mic is from the source, how far the speaker is from the mic and how far it is from the listener. Generally we want to have a "safety margin" of 6dB below MGBF to avoid resonance. This is called called MAG (maximum acoustic gain). Basically to get good gain before feedback, get the mics as close as possible to the source, and as far from the speakers as possible. Directional transducers (both speakers and mics) will net you about 6dB more MGBF in higher, more directional frequencies. This means you'll get low feedback first since those frequencies are omnidirectional. Here's the formula to calculate MGBF, just a bunch of ISL (inverse square law) calculations:
where D0 = distance from performer to listener, Ds = distance from performer to mic, D1 = distance from mic to speaker, D2 = distance from speaker to listener
Units don't matter; just make sure they're consistent.
Feedback happens when the loop gain reaches unity, e.g. level from the speaker at the microphone reaches the same level as the performer at the microphone. The funny thing about ISL is that you'll gain the same amount of MGBF from halving the distance from the performer to the mic (which is usually small anyways) that you will from doubling the distance from the speaker to the mic which is a much larger distance.
Don't put any of the choir mics through the stage monitors; it will kill your MGBF. Only run the accompaniment through them so they can hear what they're doing. Not only do vocals in the monitors introduce feedback problems, it has a psychological effect on the singers. When they hear themselves in the monitors louder than they hear themselves normally, they will have a tendency to back off a bit which doesn't help them or you.
As Dale says, foldback of the choir will kill you gbf expecially with the choir mics, IF possible are they singing infront of a band shell? This can help them to hear one another, and allow you to just have the accompaniest in the monitor
Something that has helped me with some venues is to provide some type of acoustic treatment to the area around the choir. By killing alot of the reflections I have been able to get an extra 3db or so from the chior mics.
The painful truth is that my experience is that if I can get 6 db of gain before feedback when micing a choir, I'm doing good. While 6 db ain't much, it helps. Also, my experience is that the less mics, the better. There are several articles you can find by googling "micing choirs" which will explain how to calculate the number of mics and their distance from each other, and from the subjects.