Question: Is it possible to work only on css, to set a dynamic height of the div to a proportion of full screen? 

Would like to have something like the following example

```
.my_div {
    width:100%;
    height:90%;
}

```

Answer: 
You simply haven't done the math right. There is not enough space for the `div class="user-header"` to be positioned on the RH-side. See the [**JSFiddle with borders**](http://jsfiddle.net/xVXPk/9/). 

**EDIT**: and you need to float:left the `div class="company-header"` as well: [**live demo**](http://jsbin.com/yibid/1/edit?html,output). 

Used code: 

```
<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Live demo</title>
<style>
div {
    border: 1px solid black;    /* added */
}
#header {
    width: 600px;
    height: 60px;               /* added */
    background-color: #585D63;
    padding: 15px 0px;
}
#header .logo {
    float: left;
    width: 33%;
    max-width: 180px;
    padding: 5px 30px;
}
#header .logo img {
    max-width: 120px;
    height: auto;
}
#header .company-header {
    font-size: 150%;
    letter-spacing: 1px;
    margin: 0 auto;
    width: 33%;
    text-align: center;
    float: left;                /* added */
}
#header .user-header {
    float: right;
    w/idth: 33%;                /* de-activated */
    max-width: 180px;
    padding: 5px 30px;
}
#header .user-header a {
    max-width: 120px;
    height: auto;
}
  </style>
</head>
<body>
    <div id="header">
        <div class="logo">
            <a href="index.php"><img src="#"></a>
        </div>
        <div class="company-header">
            <a title="title" href="index.php">
                <b>Hello</b>
            </a>
        </div>
        <div class="user-header">
            <label class="user"></label>
            <a class="" href="#">Logout</a>
        </div>
    </div>
</body>
</html>

```
Add class to anchor: `<a class="logout" href="#">Logout</a>`

css:

```
#header {
    position: relative;
}

.logout {
    line-height: 58px;  /* Same height as #header */
    position: absolute;
    top: 0;
    right: 30px;  /* Same padding-left of logo */
}

```

[DEMO](http://jsfiddle.net/xVXPk/19/)