Zoo Med recommends the 100 watt MVB be placed between 12-24" from tortoise height. From a purely UVB standpoint, I would think 16-18" would be a good range.I got the Zoo Med Powersun MVB 100W. It's still warming up, but the dial thermometer is reading 95 degrees. I will also use my temp gun within the next hour to get an accurate reading. Do you find big variation between the dial and gun? Mine can be anywhere from 5-10 degrees of variance. With the 125W bulb, there was a helpful guide for knowing how high to hang the bulb, I didn't see one with this 100W.