From memory, I don't think you can. However you can place an object (like a line) at the top of the subform and access that's y coordinates.
Thsi example http://assure.ly/uPXhPk has some examples of moving objects around a form, based on x/y coordinates. This example http://assure.ly/pGOS6t has script to maintain an object in a set position. I probably have some more examples.
Hopefully these can help you,