Proper use of temperature gun in ball python enclosure

[luc5810]

I am currently setting up the enclosure for my first ball python. I am using a plastic bin with a heating pad and thermostat for temp control. I also bought a temp gun to help to keep the correct temps. Should I be taking temp reading on top of the substrate or should I push the substrate aside and take readings from the base of the tub? I had tried both and will get pretty significant temp readings (i.e 88 F on top of the substrate and 92 F when I push it aside).