How can I compare heat levels from burners?
It is not a property of the stove (or the markings on it). Words like "medium heat" actually refer to the speed at which your food is cooking, and there are a lot of factors which contribute to that. Beyond the energy output of your burners, there is the type of stove (electric, gas, induction), the thermodynamic properties of your pan, the relation of pan size to burner size, how you preheated the pan, the amount of food you put in the pan, the quality of your raw ingredients, and other such stuff. So you cannot calculate it or compare it. 

The only way to "calibrate" is to observe how your food is cooking. I can only give you very rough guidelines, since the words are not (and cannot) be defined precisely. "Slow heat" is the setting at which the food can be left alone for at least several minutes without much risk of burning. "High heat" is the setting at which you end up with a very crisp/charred outside and raw-ish middle. "Medium" is somewhere in the middle between the extremes. 

Any of these will tend to correspond to a somewhat narrow range of stove settings, because when the same person cooks in the same kitchen, there is rarely too much variation in the other variables of the process (which make it so diverse in general). So you have to be able to recognize the current level of "heat" when you cook (I was tempted to say "by looking", but it actually also involves smell and hearing and sense of time). Once you see that your food is cooking at "medium", you can see which setting you are using, and after a few trys with your own stove you will know which setting to try the next time you cook.