In olden golden days, when more decisions were made by informed crews, we did the following:
Departure airfield = 100%
destination = XYZ%
Time of flight in hours x 2.5% = ABC% & is cost of carrying extra fuel, assuming increased takeoff weight due to extra fuel. If ABC% < XYZ% = tanker.
Now we don't have the % data; we just have a yes/no list. The difference could be a little or a lot and you could make an informed judgement. Now 1% or 10% difference shows up the same yes/no.