But in the end, I want to get to the point where you are, using a rough rule of thumb. I'm just taking the evidence-based route there!
Indeed, this pseudo-accuracy seems pointless.
The only relevant conclusions that taking the temperature can deliver is whether, a) there is no fever, b) there is a mild fever not requiring major concern, c) there is a major fever and extra care needs to be taken. No one in their right mind would classify these differences based on sub-degree differences in temperature.
Instead of just giving temperature, a thermometer could say "there is a 70% chance that your immune system is more active than usual" etc
Of course, small additional data points like "i'm exhausted and coughing more than usual" increase the confidence of the result by orders of magnitude over just temperature...