There are multiple ways to calculate the need for tankering fuel, it also depends on which type of aircraft you are flying.
With us for example in the winter we use a different method than during the summer. The reason being that during medium to long sectors full fuel tanks can lead to induced icing and you then require de-icing at destination, which costs time and money. The total being more than the money you saved on the fuel bill by tankering.
Some many variables, and all of them becoming more important every day with the rising fuel costs. When I started we have 15 minutes of company fuel extra for every leg, that has now been reduced to 0 and variable contingency (with minimum 5 min) calculated for the destination and route.