I'm installing a 2 ton heat pump in the bonus area of my home. The trunk is going to be between 50-55 feet straight shot. Just wondering what size I should start with and how often i should step down. There is going to be seven runs of 6inch flex going to the supply registers (2) will be in the first ten feet The next (2) will be about 20-25 feet from the AHU and the next one will be about 35 feet from the unit and next will be 10 feet from that one and the last one near the end. The trunk is going to be round and insulated on the outside. Also I plan to put the supplies in the ceiling and re 2 returns in the wall. Is this OK or should I put the returns in the ceiling and the supplies in the wall. Just not sure how to get the larger return lines back down through the rafters unless i box them in some where.