You haven't given much information, so the advice that I will give is to put the microphones where the sound is good. This will take experimentation and listening, but it is the only way to proceed.

If you told me that you had a group of 30 people singing in The Music Recital Hall at the University of Montana, I could give you exact placement, because I have experience with that choir and room. If there are 15 people in the choir, then the placement is different.

Also, I would probably use the Pearl Stereo mic as the nearfield and use the U87's as the ambience microphones set as an equilateral triangle within 20 feet of the Pearl array.

Set the U87's as figure 8's with the null facing the choir. :)

Originally posted by Richard Kuschel:
Also, I would probably use the Pearl Stereo mic as the nearfield and use the U87's as the ambience microphones set as an equilateral triangle within 20 feet of the Pearl array.

Set the U87's as figure 8's with the null facing the choir. :)

Is this a common practice ~ to have the ambience mics' null towards the choir? You really don't want to pick up the choir itself, even if they're far enough away that phasing won't be an issue?