Navigation Resizing Problems

Hello! I am really liking this theme. So far everything is great. I’m having a bit of a problem with the menu. I like how it is responsive for mobile, but, I’m having issues with it now that I have a few more items.

When I view it at 100% the items are on a 2nd line, and I don’t know how to make the menu stretch to the width of the container. I’d like for it to be in the center…and width of header.
Screen shot: https://bit.ly/1cVEGjJ

Next, when I am zoomed out, it floats to left and is very long..
Screen shot: https://bit.ly/1cPe58b

I’ve tried to tinker with the CSS as best as I could but can’t seem to get it right. Any tips? I looked @ the snippets but the code available didn’t work.

Thanks, any help is greatly appreciated.