$stylevar[formwidth_usercp]
The user control panel form width StyleVar does exactly the same job as the form width StyleVar $stylevar[formwidth], except that it allows the width of the navigation panel in the User Control Panel to be taken into account.
The red arrow shows the width controlled by $stylevar[formwidth_usercp].For a discussion of the values this StyleVar will accept, see the page relating to the Form Width StyleVar $stylevar[formwidth].