Started this job about a month ago. Two package units, 15 ton York, and two split 7.5 ton Yorks going in.
We got the duct roughed in for the auditorium area, low voltage wiring run and regrigerant lines for one of the split sys run through ceiling,too.
Tried to keep things moving so we wouldn't be boxed in by walls, etc. so hung the returns even before the wall was put up. Unfortunately I put in 1 rather than 2 return heads and STILL had to hang duct in that closet.
The offset is to get both system returns inside each closet, and equal off the corner of each closet.
Came out OK though, the two returns have to be symetric, and were measured off blueprints. when you sight down box in ceiling both returns line up on either side, and are equal off corner of wall also.