I had been using Neil Martin's dial gauge holder which can mount a dial gauge in X,Y and Z - soliforum.com/misc.php?action=pun_attachment&item=77&download=1.
However since I changed the stock Solidoodle extruder to the MK3 thingiverse.com/thing:40711 I can't use it anymore. The Solidoodle is prone to backlash in Y unless the belt tension is just right, and a dial gauge that can be mounted in the Y direction is essential. It is hard to know if you have it calibrated correctly just from measuring prints.
Newer Solidoodles are shipping with a zip tie around the motor to hold it more securely to the X carriage. This won't work with those, so I will need to figure out another version that will since I want to add a zip tie to mine as well.