A Question of Ounces and Yardage

I am doing a shawl from Instant Expert: Knitting by Ros Badger. It states that I need 10 x 2 oz. balls. Unfortunately it does not give yardage and I did buy what it said yet I ran short in spite of …
How can you work yardage when the pattern doesn’t give it and all yarns are different yardages?

I usually got to www.yarndex.com and see how much yardage is in the pattern’s yarn. Then I make sure that I have enough in my yarn.