Center align nav bar stretches child menu items

Hi together,

in order to center-align my footer menu, I added

.nav ul {
margin: auto;
width: 745px;
float: none;
padding-top: 1em;
}

to my style.css. It worked perfectly.

However, the child (and sub-child) menu items are stretched now. Please see this picture for clarification.

How can I get the child menu items back to their normal length whilst keeping the main menu center aligned?

Thank you very much for any help on the issue.

Josef