If the room is cool and the rack is uninsulated, then it may not be enough during cool Spring weather.
The thing about belly heat is that it can work in all cases.
So, if you want back heat to work for you, insulate the back of the rack with Reflectix insulation, and maybe the sides too. In fact, if you can, run the Reflectix down the inside back of the rack, and tape the Flexwatt to it.
Also, for back heat, I would think one strip (run vertically) of 11-inch-wide Flexwatt is better than the 3-inch. Actually I know this for fact, as I have a back-heated rack that started with 4-inch-wide Flexwatt and ended up with 11-inch-wide after much struggling to get the temps high enough in the back of the boxes.
Snakes do best with a range of temps. Keeping the back of these boxes too cool is bad in the long run. So is not having a much cooler area. Do whatever you have to do to get a warm AND cool area in the cage. I have no idea how to do that without some size to the cage. For me, it is the CB-70 size box or nothing.