I use racks for all of my kings and corns. They are heated via thermostatically controled FlexWatt heat tape to provide a constant 85*F at the back of the rack...the "cool end", or the front of the rack, has no supplemental heat or light source. Ambient lighting and temperatures provide plenty of light and warmth. This gives me a daytime temperature range of 80-90*F during the day, and 70-85*F during the night. Wintertime temps vary due to ambient temps. of my house...70-85* day and 60-85* night.
For my BP and my BCI, I use unregulated T-Rex Cobra heat mats on the sidewall of the viv, which provide a ground-level spot heat of approximately 92*F(inside the warm hide), and an ambient temp of around 85*F. I use a 15watt full-spectrum light during the day to provide an exterior basking spot, and they each have branches upon which they can climb to get closer to the light for higher temps. During the night, I use a 15watt red incandescent to provide minimal heat, but prime viewing opportunities.
When all is said and done...my supplemental light sources are not there to provide heating for the cage, they provide small basking areas.
Oh yea...my BCI is in a 29gallon Critter Cage, and my BP is in a 20long Critter Cage...FWIW.