Positioning a div within a parent div using auto margin or % - css

Question

I was under the impression that when using % or auto for margins on a div contained within another div the position would be calculated in respect to the parent div.
So if I have a div with height: 50%, margin-top: 25% and margin-bottom: 25% the box should centre vertically within the parent div.
When I do this though the div centres on the page not the parent div.
The CSS
div#header {
width: 100%;
height: 100px;
margin: 0px;
position: fixed;
}
div#leftnavigation {
height: 50%;
margin-top: 25%;
margin-bottom: 25%;
float: left;
}
And the HTML
<!--Title and navigation bar-->
<div id='header'>
<!--Left navigation container-->
<div id='leftnavigation'>
<p>efwfwgwegwegweg</p>
</div>
</div>
In my case there are other divs floated to the right of the one detailed above, but any one of them behaves the same way. I'm assuming I'm doing something daft but I've been over all the other questions I could find along these lines and still can't figure it out.
EDIT
Here's the JSFiddle as requested http://jsfiddle.net/ChtVv/
UPDATE
I've tried removing the margin constraints and setting the leftnavigation div to height: 100%, this works so the issue is with the margin attribute?

Answer 1

The reason it didn't work is that percentage-margins are percentages of the parent's width, not its height. You can tell this by using margin-top: 25px; margin-bottom: 25px;, and also by increasing the width of the right-panel in jsFiddle.
In all cases % (percentage) is a valid value, but needs to be used
with care; such values are calculated as a proportion of the parent
element’s width, and careless provision of values might have
unintended consequences.
W3 reference

Answer 2

CSS is tricky!! :D
This is a borrowed technique to centre vertically and horizontally, but it would involve changing your HTML and CSS. I am not sure how flexible you are with your code:
CSS:
#outer {width: 100%; border: 3px solid red;}
#middle {width: 100%; text-align: center;border: 3px solid green;}
#inner {width: 200px; margin-left: auto; margin-right: auto; text-align: left;border: 3px solid blue;}
/* Courtesy: http://www.jakpsatweb.cz/css/css-vertical-center-solution.html */
HTML
<!--Title and navigation bar-->
<div id='outer'>
<!--Left navigation container-->
<div id='middle'>
<p id="inner">efwfwgwegwegweg</p>
</div>
</div>
You can build upon this to achieve whatever you are after!
fiddle: http://jsfiddle.net/pratik136/ChtVv/2/

Answer 3

Ok, so there are a lot of reasons why this would not work.
The main reason would be that your container has position:fixed;
When adding position:fixed; to a element, it no longer reserved it's space in the DOM and won't contain it's children.
I have made a example of the best way (in my Opinion) to center your child both Vertically & Horizontally
Here is a demo.
Demo
And here is the code.
<div id="container">
<div id="child"></div>
</div>
#container{
width:100%;
height:500px;
background:#CCC;
margin:0;
}
#child{
width:50%;
height:50%;
background:#EEE;
position:relative;
top:25%;
left:25%;
}