Every spring my company gets a bunch of calls to record this type of event for fundraising for the local county schools. The best results seem to be when we record about 2' behind the conductor, and about 1' over his/her head. We've done this in some horrible sounding rooms (gyms, audituriums, cafeterias), and it seems, to me, at least, that the conductors perspective is usually the best spot. ymmv.