Heating element wattage? AP T8 w/ 8'' screen cutout

[dkspftw]

Just purchased an AP T8 cage with the 8'' circular screen cutout included.

Going to hook up a ceramic heating element in the cutout, but what kinda wattage should it be? I use a 100w in my 2' tall terrarium that my other BP is in, so I figure that this will require less wattage, being only 12'' tall. It's gonna be hooked up to an automatic shutting off thermometer.

What do you guys think? 75w? Or do I just do 100 so it reaches the right temperature but spends less time switched on?

Thanks in advance for your knowledge and expertise.