Stihl USA features a free chart for easy 50:1 ratio fuel mixing at stihlusa.com. The website lists ratios for mixing 1, 2.5 and 5 gallon containers of 50:1 gas and oil mixtures.
Stihl recommends pouring the oil into the container first, then adding the gasoline, followed by vigorous shaking to ensure an even mixture. After the mixture has been stored for any amount of time, the container should be shaken again before fueling up an engine. Stihl recommends that fuel should never be stored longer than three months. Gasoline used in the mixture should have a minimum octane rating of 89.