Info from Robert Openshaw, elog:
http://midtwist.triumf.ca:8081/EL/021122.6953?exp=twist

The NMR signal (as seen on the scope) will drift for two reasons:
a) The frequency of the RF source in the magnetometer drifts with temperature
b) The B1 magnet current drifts with temperature (i.e. the *real* field 
changes)

If the "squiggle" on the scope drifts from the center of the trace, it could 
be for either or both of these reasons.

If you want to observe/record the present magnetic field - adjust the fine 
pot to center the NMR signal on the trace, and read/record the displayed 
number.

If you want to adjust the magnet to the correct field - adjust the fine pot 
to set the displayed number to the desired field, then adjust the magnet 
current to move NMR signal to the center of the trace. 

(Without using the fine DAC you probably can't get it to the precise center 
of the trace, so either use the fine DAC, or get it as close as possible with 
the coarse DAC and then adjust the fine pot of the magnetometer to center the 
NMR signal, then read/record the number.)