Yardage and Yarn Substitution

I hate having all the odd balls but I hate even more when I run out of yarn. So I always buy an extra skein for a project when using other’s pattern and get at least one extra skein when designing my own pattern. I love the freedom the extra yarn gives me during the design process. I avoid constraints whenever I can (one of the main reason why I love to publish my patterns independently). I know that it’s hard to know how much yard to buy when you substitute yarn in a pattern. If it calls for 5 balls of a yarn that has 100 m per skein, how many skeins should I get if I substitute it with a yarn that has only 95 m per skein? How much of the last skein did the designer use? How much allowance was included? I really wish that these information are in all commercial patterns. Now that I publish my own patterns, I try to include them whenever I can to make it easier to substitute yarn.