It depends a little bit on what model you have. For example, if you have one with a distance-to-spot ratio of 8:1, you could be 8 inches away and be measuirng a 1" diameter spot. Some units only have a ratio of 1:1 which means that you have to be 1" away to measure within a 1" diameter circle. The laser light's purpose is to help you aim more accurately when you are many inches away.
Bottom line: You will make the most accurate measurement if you are very close. You can experiment by measuring your hand at various distances . As you move-in closer, watch for the reading to become stable. At that point and closer, you'll be making measurements that are as accurate as your particular unit is capable of making.