First of all, it is much easier for us to help you if you post a URL for us to look at. When dealing with absolute positioning, it is worth remembering the following rules:
The positioned element is laid out in relation to its containing block.
The containing block is defined as the nearest ancestor with a position other than static. If no such ancestor exists, the containing block becomes the root element of the document.
If the nearest ancestor (whatever element contains the IFRAME) has a style rule of position:relative, the IFRAME will be absolutely positioned within that container.
If the nearest ancestor is the BODY element, I recommend wrapping the entire content in a DIV element, and then set it to position:relative. No offset is required to make it work. Then you should be able to use right: 600px on the IFRAME to put it where you want it. I can't imagine why you should want to do this, but you're the designer.
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