Hi Angela , i found this chart which I think will help you decide how much fabric you need for the pillows. Even though I don't usually make welts you still need about the same amounts for knife edge pillows. If you want welting I think purchased is easier sometime.
Size With Pattern Match Up to 18 " wide 1 1/2 Yards Each
18" to 24" wide 1/2 Yards Each 25" to 36" wide 3 1/2 Yards Each 37" to 46" wide 4 Yards EachJuno