The Layout Editor works in increments of 4px, so your client will be able to achieve 1000px or, 996px using settings. The Global Content Width field isn't really designed to have a specific value entered.
To accomplish a unique value, it's possible to apply it via CSS, once we know which specific level your client needs changed. Do you use Firebug or Chrome's Inspection Tool?
Either of these will help us identify where to place width: 998px;
I'd be remiss if I didn't mention that a specific width like that will ruin the site's responsiveness in mobile devices.