Accessing Xrm.Page from a webpage to a CRM form in an IFrame

var XrmPage =document.getElementById("myIframe").contentWindow.document.getElementById("contentIFrame").document.frames[0].Xrm.Page

But don't understand what Id is needed for 'contentIframe'.

I'm trying to remove a optionset item from the webpage by using

var pickListField = Xrm.Page.getControl(“fieldName”);
pickListField.removeOption(Option_Value);

is this a good option? or is there a better way.

Thanks

Gabby