Anchors: Scroll to position on page minus the height of the header

I have a webpage http://ift.tt/1KVVmy2

As you will see it has a fixed navigation. When you use the navigation it scrolls down the page for you automatically. My issue is that I need it to scroll to a position, for example the 'ABOUT' section, minus the height of the header.

If you look at the page currently you will see that because of the header it overlaps the about title.

Any suggestions or can anybody point me in the correct direction?

Thanks, Alex