The IR doesn't penetrate much beyond the surface so you will always be taking the temperature of the surface. At a vigorous boil, I've noticed it reads the same or very close to a digital probe thermometer placed in the middle or bottom of the kettle. Probably because anything at the surface isn't there for very long. When the liquid is still, like during mashing, the surface temp can be very different. The two solutions that I found are: 1. stir as taking a temperature and 2. leave a stirring spoon or spatula (preferably a matte black one) in the wort. Pull it out and shoot the IR at it. I have compared temps using both of these methods to readings from both a digital probe thermometer and an analog thermometer and have found they are accurate.
An important note though, the quality of IR thermometers varies widely. I haven't used the exact model that you stated but I have used a few different ones. The one I use now is very accurate but I have used others that can't seem to get a good temperature at all.
Hope this helps.