You need to scan the DIY forum for underquilts. Study each design. In most cases they will tell you why each thing is done.
As to temperature rating, I use this formula:
Temperature = 68 - (18 x thickness)
Temperature is Fahrenheit and thickness is inches.
To determine how much down you will need multiply the width times length times thickness of the quilt and then divide by the fill power rating of your down. Then make your baffle height 1" less than the thickness.