Increasing width of mainbody container

I want to increase the width of the mainbody container from the default 61% to 70% but cannot figure out where I do this. Can anyone please help?

Thanks
George

You need to decrease the columns width via template manager, "Layout" tab.

I don't have anything in the column position

That's incorrect. I have nothing published in either of the two left column positions. It is the mainbody/component that I want to increase the width of.

Thanks
George