The logic is like this:
Get out your flight planning data for somewhere A to B
Pick a weight (W) and look up the fuel burn and time.
Pick another weight (say W + 50 tons) and get the fuel burn and time again.
Do some sums and calculate the extra fuel burn to carry one tone for one hour. (It will be something like 4% per hour iirc).
Get the current fuel price and you can now work out the cost of air-freighting that fuel to any destination if you know the flight time.
Add the air freighting cost to the original fuel price.
If the fuel price at destination is greater than this, then it pays to air freight your own fuel to the destination.
I did this exercise years ago for an airline and Green Guard's numbers look pretty good to me.