I was told specifically not to use foil tape by the owner of the Herpstat site. He said to use clear packaging tape, which I didn't have, so I just used Scotch tape.
It is fairly consistent, depending on what I'm aiming at. If I aim it at my wall, it is within a one degree margin every time. Flexwatt, more like a two to three degree margin.
I've heard of temp guns reflecting off metal and giving inaccurate readings. Interesting.