To figure out how much yardage to buy use the binding calculator at http://www.quiltersparadiseesc.com/Calculators/Binding%20Calculator.php 
and select 2 1/2" for the binding width.

The total length calculated is the number I use for the services charged to your project.
This is the formula: (L+W) x 2 + 10

For example your quilt is 50 x 65 inches.
50 + 65 = 115 x 2 = 230 + 10 = 240. All 'per inch' charges are based on 240 inches.