I always just stand in front of the choir, pointing directly at the congregation. I also try to stand to the side of the director, if there is one, and definitely never in front of the organist. The only time I ever sort of played at the organist was during a Christmas Eve gig, where he played Joy to the World at about half tempo. I tell you, he only played slow for about 3 measures. I find that the congregation will never say you were too loud. Just tell the director to walk around the church while you play something, so they can hear you once the sound has expanded into the room. Usually, they have no idea what it sounds like from the pews.