There is no simple formula in the case of ground fires.
I once was starting engines on pushback on a 4 eng heavy and the ground crew on the intercom shouted "engine fire". Company policy was evacuation if flame was positively sighted. I questioned the fire call as to its nature "smoke or flame" and he said "just showers of sparks". It turned out to be the starter motor disintegrating and the sparks stopped after about half a minute and no evac was necessary.
We need very positive guidelines but not absolute, blind rules. In the end, it comes down to experience, communication and judgement and even then there are no guarantees.