It can be quite tricky when substituting yarn types. Cotton doesn't stretch the way acrylic does. I can't say what a wool blend would do. The easiest way to do it is with the yarn specified in the pattern directions.

I would try it and see what happens. You can always frog it and go buy the cotton if it doesn't work.

"Seven days without crochet makes one weak"
http://yoursandi.blogspot.com

Thanks for the advice. I will probably just go buy the cotton yarn.