When you work in Atom with split layout, every time you should change the width for the current active tab. This plugin can do this job for you. Just define the percentage value of the width and let the plugin do it for you.
Open Atom Settings, go to Packages in the left hand sidebar, and press Settings on active-tab-width to open the settings for the package.
*Min: 70% *Max: 100% *Default: 90%