A monitor is going to be essential for the artistic choices in image framing and planning, but not for perfect calibration. A large monitor can be used when blocking a shot and arranging what it will look like essentially. With this you can get a pretty clear idea of how the image is going to end up looking in the final print. The monitor is not a clear way of seeing exactly how sharp the image is. For actual focus the best way to calibrate is to do it through the viewfinder. It is hard to get into the focus object, or focus card, on the monitor. Instead the monitor is simply to be used to get a general idea of the image itself instead of the finer technical areas.