To elaborate a bit more. If you are running an 8" duct off the supply, would you do a quick 90 degree towards the destination or do a gradual 90 degree?
I would guess a quick would be better because you have less cfm loss per foot. I could be wrong.
The 8" will hit a wye to two 6" ducts which run into the register boxes.
The registers are 5 foot from each other. The more important register (deeper in the room to be air conditioned) will be straight off the wye. Maybe 10 foot straight from the wye versus ~12 - 15 foot off the other side of the wye.