Change Vaadin Board Row

Hi all,
I’m starting to use Vaadin 10 for to create a web application.
In order to use <vaadin-board> and <vaadin-board-row> feature I’d like to know if is possible to change the
flex-basis value that is auto set in the div style.

I need to create a row with a vertical menu (so width=15%) and the application content in the other side of the page.
Is this the best approach? Is possible to change the column width?

Can somebody help me how to implement this scenario also because it seems that I cannot have more then 4 columns in a board row.

Thanks
Gianluca