Practice make perfect with your gun, I have run mine, (a 29.99 HF Special) along side a 200.00 calibrated unit and they consistently read within a half degree of each other, much to the chagrin of the other gun owner.
Understanding how much the reading zone increases in size as you move away from the target is the key to accuracy. A larger zone causes it to take in information from surrounding area and objects. The laser is approximately the center of the read zone, but the read zone can be several inches in diameter, so be close to your objective and consistent. The boiled water is a good way to check your calibration with consideration to your altitude. Again with the distance from the surface of the water though, steam is not the same temp as the water and will affect your readings.
Another great use is to monitor your brakes by reading the temp of the drums after a run, if one side is hotter than the other, attention might be warranted. Check your head temp as well, over each cylinder, and the exhaust manifold at each cylinder is a good troubleshooting tool. Benchmarking that information when the car is running good, will be valuable if you suddenly suspect you have an issue down the road.
Have fun with it, in my opinion, with its low cost, dependability, and portable size, it is one the best diagnostic tools you can own.