Defining the % space of columns (in a row)

Hi Tagdiv support team,
I’ve been modifying the prebuilt “Magazine Pro” website, and I’ve been stuck on the Header.
I’ve notably taken out the search functionality (successfully), as well as changed the background image.
The remaining issue I have is as follows: the header is now composed of two columns: one of the logo on the left, and one for the menu items on the right.
As I have quite a few menu items, they unfortunately spread over two lines. I would like to reduce the space for the column on the left for the logo (especially since there is quite a bit of unused space already!).
But I can’t seem to find the place the define the % of space for each column.

Could you kindly point me in the right direction? Here’s a link to a screenshot where I circled the red space I would like to reduce/get rid of.
Thanks!