For my current milk can heater I just use its built in control. The stuff I am growing isn't fussy enough to bother with an external thermostat. With the external thermostat, you need to have your heater on the "high" setting for energy and the dial turned all the way as warm as it will go. Otherwise at 70-80F your heater's internal thermostat will interfere with the functioning of the external one.
You might consider getting the greenhouse to 60F and then put heat mats under your tomato trays. Until they are a few inches tall, they are much more concerned with soil temperature than room temperature.
