Pretend you cut your enclosure in three equal parts. So if you have a 3ft long enclosure each part would be 1ft. Pretend the left section is your cool spot and the right section is your hot spot. The middle portion is the "mixing" spot. Take the cool spot and measure it right in the center and do the same for the hot spot. The mixing spot will be cooler then the hot spot but warmer then the cool spot, so that section you do not have to worry about measuring. You should measure the centers of each portion (this is also a good marker for where your hides should go).