This is strongly dependent on company, aircraft type and route structure.
On my current type, 1% of index difference makes up for the costs of carrying 1 hours worth of extra fuel. So for example if the destination is 1,5 hours of flight time away and has a fuel index of 102 while the departure field has an index of 100, it is starting to get economic to tanker for both legs on the departure field.
On other types, there are tables of flight time vs. required fuel index for tankering. And other types will have the flight log show figures for savings/extra costs per unit of fuel tankered.
If I were in Your shoes, I would contact Your companies accountants for guidance, as they should know Your companies cost structure.