How can I make ajax calls within Custom Module's management console?

I’d love to be able to call functions from within my Custom Modules management page. But I’m struggling to work out where to define the php functions and how to work out the URL to call them.

I thought I’d just need to define a function in my modules mcp class and call it using something like this

$vars['ajax_url'] = 'C=addons_modules'.AMP.'M=show_module_cp'.AMP.'module=document_editor'.AMP.'method=ajax_function';

But I realise that’s not right. I’d welcome any suggestions on how to structure my module to allow me to do this.

Thanks

Andrew