How High Should the 2H Lock Signal Be?

New users of NMR spectrometers will often ask: How high should the ²H lock signal be? This question is asked while they are shimming the magnet. They want to know if they have "shimmed enough". There isn't really an answer to the question. The height of the lock signal depends on all of the following: which solvent is used, the amount of solvent in the NMR tube, the lock power, the lock gain, the lock phase and the magnet homogeneity. The lock signal is high enough when any further shimming cannot improve on the height.