I think it's valid. The thermal doesn't know if it's day or night, just the differences in heat signatures. I was just switching over a Steiner S35 to another rifle this week and went out to the desert to finish sighting it in. I initially bore sighted it using a cup of ice water at 25 yards and a laser bore sight. That has worked the best for me for me by putting the laser on the cup and moving the crosshairs to the center of the cup. Initially the AR500 gong that I took with me was warmer than the outside temp of 65 degrees and I could see it clearly, but it only took about 20 minutes before the temps were even enough I had a hard time finding it at 100 yards. I've since decided to take an ice chest with me and alternate the gongs in and out of the ice chest when I need to do it again. I've used black tape on cardboard, foil on cardboard, heating up a quarter, but the steel gives that satisfying gong without me having to walk out 100 yards to check every hole. In the house in the daytime I can see the dogs footprints on the cool tile when he walks, it's that sensitive day or night.