if you are using flexwatt to heat, i do NOT recommend the Zoomed reptitemp 500r.
The ONE part of this entire thing you do not want to skimp on is the heating element control. Spend the 80 at www.reptilebasics.com with Rich and get a ranco or johnson controls unit prewired and ready to go. Rich will also help answer all of your questions.
The 500R is only acceptable to me in back heat applications when used with flexiwatt, and even then I wouldn't trust it. The fluctions i experienced with it were far to great for my taste.
Unregulated zoomed heating pad: 122F
regulated zoomed heating pad with zoomed thermostat: 114F peak (78F low)
Regulated with johnson control unit: 94F-96F.
those are actual numbers i recorded with a digital thermometer (accurite).
Rich also has another great heating unit http://www.reptilebasics.com/ultratherm-heat_pads/ . Im using one of these for my leopard geckos. They do peak a little higher than I'd like for ball pythons (although it could probably use a bit more ventilation). My temp gun routinely reads around 97 on the glass below the substrate. (they are in a display type cage, whereas all the snakes are in racks).