Website width Issue

I’m having the same issue. (website is currently ‘down for maintenance’ but its ats-sales.com)

to resolve it i’m looking for css that makes everything a proportion [eg. 80%] rather than fixed pixel quantity [1500 px]
but nothing i’ve done is working properly

i’m trying to make the container X percent justified right. i feel that if the container auto expands then the navigation [right menu] will automatically be pushed to the right