Hi, I do it the following, but that takes a lot of time.
I set the width to ZERO(leave it blank). then I center the Link-Text finally I set the padding (left/right) to 20px for example. now, you need to see the live page if the navigation fits. its try and error. the navigation is not to see in dreamweaver :-(
maybe you wish to have this flexible width on each link, and automatic fit to main width of the navigation.