This question has been studied extensively since the mid/late 1800s (no, not a typo). You will therefore be able to find graphs of the relationship between estimated fat and carbohydrate oxidation and exercise intensity in practically any undergraduate exercise physiology textbook and all over the web.
Generally speaking, however, in the fasted state (i.e., no recent food intake) we use mostly fat at rest, with a bit of a jump up to closer to a 50/50 mixture during low intensity exercise. From there the balance shifts non-linearly more and more toward carbohydrate, reaching 100% (apparently - see my previous posts about this particular issue) as the intensity approaches VO2max.
Although that is the general picture, exercise intensity is not the only determinant of the rates of fat and carbohydrate oxidation. As indicated above, nutritional state is important, with recent (i.e., previous 6-8 h…yes, we studied this time course back in my UT Austin days: Exercise metabolism at different time intervals after a meal - PubMed) carbohydrate intake increasing carbohydrate and decreasing fat utilization. A person’s habitual diet, fitness, age, sex, and probably genetics also all play a role.
What the above means is that it is very difficult to accurately estimate how much fat/carbohydrate a given individual oxidizes during exercise under a particular condition. As well, there’s not much “actionable intelligence” in such data for an athlete. Thus, although software exists that attempts to provide such estimates (and I was encouraged to develop such a chart for WKO4), this is really more of a sales/snow job than anything else.