Just for my own clarification I thought the split tempature, or as I seen others call it spread. Was supposed to be 18-22 degrees, my tech told me 20 is the bulls eye.

And like you said 78 degree inside temp on a 100 degree day fits what I've been told just fine.

Also, if that bedroom noone sleeps in is too cold, maybe its the closest to the unit.

How could one balance the air evenly when you have a room with a short duct run, say 5' or less to the supply. A dampener might whistle. Can you undersize that duct??