The way to arrive at the optimum background is to set up your subject and bring a number of sheets of different colors and values. Not to chromatic if you want the background to remain back.
Try each behind your model and see what looks best. Many artists have said "paint from nature." What they mean is work from reality, not from your head.
If it looks good to you and achieves your purpose compositionally then it's a keeper.
Trying to do this out of your head is an inefficient use of valuable painting time, if you hadn't already noticed.
