Logo centered / menu left right

Hello,

I’ve found the way on your support answers to center my header logo with a left – right menu. But I guess stg is wrong in the code and I don’t manage to fix it : the space between the logo and the first / last item ot the menu is not the same.
Here is the code : the “pb” comes from the margin-right: 330px. Please find below the credentials and a print screen, I’d like 80px on the right too.
Tx a lot guys for your help !
Marine

 /*Centered menu*/
.av-main-nav li:nth-child(3) {
    margin-right:330px;
}
#header_main_alternate {
    margin-top: -50px;
    z-index:9;
}
.logo {
z-index:9999!important;    
max-width:250px;
}