According to this website:
http://www.airscapes.com/HLTHQST/HLTHQST7.HTM
you can substitute canola oil for butter or margarine as follows:
For 1 cup butter, substitute 14 tablespoons canola oil.
For 3/4 cup butter, substitute 10 tablespoons canola oil.
For 1/2 cup butter, substitute 7 tablespoons canola oil.
For 1/4 cup butter substitute 3 1/2 tablespoons canola oil
Hope this helps!