I don't know if this helps but my breast cancer surgeon told me the aim is 1.5-2mm of negative margins. I personally would have preferred wider margins and deal with the cosmetics later if necessary. But my post-op biopsy showed clean margins of 1.5mm and two oncologists were happy with it. So your 0.3m looks even better. [I'm phobic about too-narrow margins after a friend's new husband died from skin cancer and M.D. Anderson concluded the original surgeon from a different facility removed a few millimeters too little. So I totally get your question.]