Keep the cutting length first
Raw fabric length is the amount needed for the cuts in the plan. It comes before any optional extra fabric and before shop rounding. Keep that number: it's useful for checking fabric you already own, even when the shop quantity is larger.
Round up to the amount the shop sells
There are 36 inches in a yard. If you need 20 inches of fabric, that's about 0.556 yard. A shop selling in ⅛-yard increments won't cut that exact amount, so round up to 0.625 yard, or ⅝ yard. Rounding down to ½ yard would leave you 2 inches short.
Add extra only once
In the binding calculator, the extra-fabric percentage is applied to the raw cutting length before purchase rounding. The result lists that extra length separately. If you've already allowed for a concern in the measured dimensions, don't automatically add the same amount a second time at checkout.
Watch the next purchase step
A small design change may leave the shop quantity unchanged or push it into the next increment. Compare the raw length and the rounded yardage when deciding between options. The rounding remainder is spare length from that purchase step; it isn't a guarantee that you'll get another useful piece from the fabric.