Adam,
Editor.FullPage is close to what I want but my example was simplified to make it easily readable. My real page has a header div and a sidebar as mentioned in the original post. Using Fullpage mode covers these up.
If I set the parent div to 500px then resize the div with script using document.documentElement.clientHeight-100 or similar the editor has problems when switching to html mode (edit area is tiny, toolbar and footer fills the screen), and also if the user makes the browser narrower causing toolbars to wrap it cuts the bottom off.
In simplified form I want: editor.fullpage - 100 pixels for a header. The sidebar is not important as 100% width works fine. So my code is:
<div style="height: 100px" id="header">Header will go here</div>
<div style="height:500px">
<CE:Editor id="Editor1" runat="server" Height="100%"></CE:Editor>
</div>
Thanks