LC,
Assuming that OB's figure of 5% is good for each hours extra fuel burn (sounds logical) then we would say that if destination was 4 hrs away then for the fuel tankering to be economical the destination fuel would have to be 120% of the departure price.
If doing a multi sector day from A via B to C where the prices were 100% / 120% /160% then the economics would be favourable if the comparison price were 160/120% = 133%.
Just what we do!
idg