How can I make a document into two columns that take up a 70/30 split of the page instead of 50/50?

I can control the spacing between and where the text stops within each column using the ruler but I can't figure out how to move where the spacing between the columns is centered. Right now with the column spacing being centered in the middle of the page, it seems I can only make a 50/50 split with the columns.

I want the left column to have about 70% of the width, and the right to have 30%.