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In my experience with a variety of temperature measuring devices and gauges, including several brands of IR guns, the normally set EM of 0.95 works fine. If you are seeing what you believe are incorrect temperature readings your IR gun may be defective (I've had a few that were including a $400 one) or your autometer/auto gage my be defective, or both are giving correct readings because the surface temp and inside temp are not the same. My recommendation for how to approach this is as follows... Get the car completely warmed up, summer is good because it will be about as hot as it would ever normally get. Then park it in the shade, open the hood, turn the car onto accessory so the temperature guage(s) are on and immediately take readings (and write them down) of your temperature guage(s) and also with your IR gun on the spot where the thermostat is, the spot where the temperature sender is, the back of the heads, the water pump body, and any other places you want to know the temperature of. Don't let things sit more then a minute because it all starts to cool down and will fairly quickly all start to cool and equalize to the same temperature. Then Wait 10 minutes and let everything (engine, radiator, fenders, etc) settle into a nice quiet big lump of steel, water, and other stuff that is now approaching the same temperature inside and out. Take more readings. Wait another 10 minutes and take more readings. By the thrid set of readings (about 20 minutes after you shut it off, pretty much everything will be at the same (still pretty hot) temperature. At that point you can compare what your IR gun is telling you to what your autogage/factory gage readings are. I have found this to work very consistently based on doing it several times now with my 1957 tbird. I have/had several of the HF AMES gauges and compared them to my much more expensive gauges and WHEN THEY ARE WORKING RIGHT they are within a couple degrees of each other. I did have bring one back and exchange it because it just wasn't reading right. My dad had one out of warranty and it also didn't seem to be getting correct readings so threw it out. They are cheap so you might want to buy a second one to crosscheck the first. I have also found that while they may seem to match other thermometers at "room temperature", they fail to properly read at 200. I have a 180 thermostat and on moderate/warm temperature days when the car is fully warmed up if I shoot the thermostat housing with the IR gun it says right at 180. On really cool days or really hot days you can't count on the thermo housing being right at 180, it might be too cool or on a hot day running a little higher.
230 to 240 is HOT. When you opened the hood did the engine feel hotter than normal? When you shut it off did it burp or gurgle coolant? Many cars, when shut off when they are actually that hot, will boil coolant inside the head and you can hear it boiling and gurgling around inside. I had a 86 Chevy that was only running about 215, not even into "overheating" territory and when I shut it off in the summer I could hear it gurgle for a minute or two and sometimes it would diesel and refuse to stop running and I had to put it in gear to get the engine to stop.