As a pattern designer - it's my job to accurately calculate the amount of yardage needed for every pattern I write. You wouldn't be very happy to purchase a pattern and the fabric only to find out you don't have enough (or way too much) fabric.
To help me with accuracy - and save time - I've created some tools I use when I'm calculating yardage and cut pieces of fabric. These tools are in the form of Charts and Guides and I refer to them all the time.