If you are fitting a round Victorian-style door knob, please ensure there is sufficient distance from the edge of the frame to grasp the knob and turn it comfortably.  To facilitate this, if using a barrel-type door latch, the back-set should be at least 75mm.  Do not confuse the back-set distance with the overall length of the latch.  A latch with a back-set of 75mm will probably be around 115 / 120mm in length.