You can estimate how close to the event horizon the disk is based on how broad the spectral lines are. The part of the disk that is coming towards you will be blueshifted and the part of the disk that is rotating away will be redshifted. From that (which is independent of the overall depth of the potential well) you can figure out how fast the disk is rotating. And from that you can figure out how far away the disk is from the black hole.