Aside from a few “hard” limits to the iPad’s use cases (e.g. compiling, multi-interface audio recording), I think the only rule of thumb on the iPad/Mac division is to follow your instinct. In other words, don’t elect to do a specific task on iPad/Mac because of stereotypes or others’ words. Do the task on the device whichever your mind tells you to be right.
True, it sounds more suitable to use iPad as a reading device, but I often find myself triaging my read-later list on Mac because it’s more efficient with more items on the screen and the full power of keyboard shortcuts. On the other hand, while it sounds awkward to produce a Numbers worksheet on the iPad, sometimes I prefer to do so rather than reaching my Mac because the iPad brings the work product right under my finger; the intimate feeling can help me to focus.
Sometimes, I begin doing something on the Mac and then switch to the iPad for some totally arbitrary reason — say, I want to stand and move. I might have save some time if I stayed on the desktop with the more powerful version of softwares, but the time loss was made up with the efficiency boost brought by my psychological ease.