Its hard to calibrate sometimes because of the different temperature ranges of everyone's reptile room. Most top breeders have thier snake rooms at 78-80F ambient, so all they have to concentrate on is a 90F hotspot! Piece of cake. But that doesn't help the people that have one snake, one cage, in a room in their house that's 68F.
The way to combat this is to get a heat pad, and put it under varying lengths of the cage. Play around with it for a week, measuring the temps with a digital thermometer. 80F cool end, 90F warm end. Just play around with it and you will get it. a light bulb CAN be used to get the ambient to 80F, but its not advisable.
Keep asking question. You're on the right track! 