I'm not quite sure if I understand your question 100%, but just as Racil put it, you want to create a css, and set it to a percentage or with pixels...
If you want to add some transformations, I use this code on my website:
nav li a:hover, nav li a.current {
color: #0099CC;
-o-transition: background 0.3s linear 0s, color 0.3s linear 0s;
-webkit-transition: background 0.3s linear 0s, color 0.3s linear 0s;
-ms-transition: background 0.3s linear 0s, color 0.3s linear 0s;
-moz-transition: background 0.3s linear 0s, color 0.3s linear 0s;
transition: background 0.3s linear 0s, color 0.3s linear 0s;}
The reasoning behind all the different transitions is for the all the old browsers that do not accept the transition code.
*Note: You will need to supply more nav info in your css code, the above is to only make it have a transition effect..*