Ah, now the light comes on.
Put *that* way BOAC it makles more sense.
I was assuming that heavy transport was using around 4% extra which could be significant on a long haul flight, but reducing to a thimble full per tank on a Cessna 150 and, as such, was so fractional as to be not worthy of consideration....and that somewhere along the diagonal line between the two aircraft styles, it started to make the bean counters think, "Errr, hang on a minute"
So aircraft X with full tank capacity of 100 gallons, goes on a two hour flight at 10 galls per hour, needs 25 gallons (to include 30 minutes reserve) will have "tankered" 75 gallons around the sky for fun - and will have burned an extra 6 gallons (4% of 75 multiplied by 2 [hours of flight]) in doing so.
So it will land with 69 gallons in the tanks and will have burned a US$30 hole in the profit margin in doing so? (assuming US$5 per gallon of avgas).
If renting the aircraft "dry", I would consider that already significant (certainly on my own budget just now).
Going one step further then, if the hourly fuel burn is reducing (as 4% of less is less than 4% of lots) there must come a mathematical time where it is cheaper (all things being equal) to fly on for another hour or two than to land with lots of spare fuel and "top off" the tanks.