.This is my formula that I use:
To calculate the width, decide what you want the finished width of the binding to be and then multiply that number by six and add 1/4” (the 1/4 accounts for the turn and height of your batting, adjust up or down for thick or thin batting).
For example:
1/2” (width of finished binding) X 6 = 3” + 1/4” = 3 1/4”