I use Sheldon Brown's Gear Calculator to determine "Gear Inches" for a particular bike. I print the chart out and tape it to the top of the stem so it's easily visible when riding. The chart allows me to easily determine which chainring-cog combination is next in the shifting progression. Many times different chainwheel-cog combinations will result in similar gearing. It's too much trouble to memorize these similar combinations. The chart will help a rider shift in a progression that minimizes duplication.