You set it hours afterwards measure it. I like using smaller wattage bulbs, makes a great temp gradient, and even heating. Ive never had a problem in several years with any of the floods even those with the center hexagon burning animals, yet Ive seen others attribute them to doing so. They have to be set to be evenly heating a surface no matter what, as well I always use multiple bulbs on any animal that the hotspot is not big enough for one bulb to evenly heat its entire STV size, thats how they get burnt, from too small of a hotspot and not enough area to warm themselves evenly, it wouldnt matter almost how much wattage it is, if the animal spends too much time under it, they will burn.