That sounds about right. I had a similar setup and I decided to get rid of the heat tape and invested in an RHP. Personally, I think for the pvc cages, RHP is the way to go. If I had it to do over again, I wouldn't even bother purchasing the heat tape. The heat tape just isn't very efficient in the amount of heat it needs to put out in order to get the enclosure where you want it. The RHP on the other hand, is very efficient, in that it puts the heat right where you need it.

My suggestion would be to ditch the heat tape, and get and RHP (regulated with a thermostat, of course)