At the substrate.

Not through the screen.

The closer the better. They are not as accurate the farther away you go.

Also. If you have a thermostat in the house, hit that with the temp gun. Read the temp on the thermostat and compare. Some temp guns can be off a few degrees. Don’t sweat a degree or two but adjust from the home thermostat. In other words. Home thermostat is 68 and temp gun reads 69, subtract one degree from temp gun reading in tank.

Depending on how the CHE is pointed there can be temp variance.