The marks on the pictures above and below are linked. If the mark on the top is on the inner rim of the frame but the mark on the bottom is not on the inner rim of the frame, check that the unit is levelled with the spirit level. Probably the unit is hanging crooked on the wall/bracket. If you have checked this and the images are still not flush, then your unit is working within tolerance and the readings are still usable. But make sure that you average the crosses at the top and bottom. i.e. you move the cross on the top image so that the centre of both crosses (top and bottom image) marks the edge of the glass or centre of the pupil.