How to make an input width relate to label width?

[Visc]

I have a \[code\]label\[/code\] and an \[code\]input\[/code\] side by side in a 540px wide \[code\]div\[/code\]. the \[code\]label\[/code\] contains a domain name, say: http://domain.tld/ and the \[code\]input\[/code\] will have the URL slug typed into it. The problem is that the domain name can change (and will change) depending of user so that means the \[code\]label\[/code\]'s width will also change causing the \[code\]input\[/code\] either to be too little in width or too big in width. Which in first case will result in ugly design and in the second case result in a broken float. Does anyone have any tricks or tips for me on this? Or do I have to ninja-javascript this thing? All help is appreciated.