I'm assuming this is what you're talking about: http://www.amazon.com/Zoo-Med-Hermit...it+crab+heater
The thermometer is crap and won't tell you the temp of the heat pad, you'll need one with a probe.
The heat pad doesn't say how big it is, but it's probably way to small for a BP.
I'd suggest buying one of these instead: http://www.amazon.com/Zoo-Med-ReptiT...it+crab+heater
Or even better: http://www.reptilebasics.com/ultratherm-heat-pads
and also buying one of these: http://www.amazon.com/Hydrofarm-MTPR...ostat+heat+pad
or even better: http://spyderrobotics.com/index.html
Heating an enclosure doesn't come super cheap, but can be life threatening if done incorrectly.
Edit: this is the thermo/hygro I recommend
http://www.walmart.com/ip/Acu-Rite-I...mometer/896347