To create a fully flexible design, it is important to code everything at 100% capacity. You can test this by following the code provided in the DEMO link.
This demonstration aims to illustrate how you can achieve this level of flexibility.
CSS
body { background: #ccc; margin:0; padding: 0;}
header {background: #fff; width:100%; float:left; height:60px; position:relative;}
nav {background: red; height:30px; width:100%; float:left; margin-top: 30px;}
.logo {
position:absolute;
width:150px; height:80px; background: #fff;
border-radius: 0 0 3px 0; left:0; top:0;
}
.triangleDown { width: 0; height: 0; border-left: 50px solid transparent; border-right: 35px solid transparent; border-top: 81px solid #fff; left: 98px; position: absolute;}
HTML
<header>
<div class="logo"></div>
<div class="triangleDown"></div>
<nav></nav>
</header>