Surely the KEY aspect here is not so much HOW MUCH extra you take
but more HOW LONG you carry it for!
It's both, actually. Carry more overage, you'll burn more of it getting it from A to B, regardless of the leg length. If the overburn on tankered fuel is 5%, then you'll burn 500 lbs of that for a 1 hour flight....if you're carrying 20,000 lbs extra, then of course you're burning 1,000 lbs of that to tanker it. So yes, it's both the legth of time you're carrying it, and the amount. Speaking in general terms and rules of thumb, of course. As stated before, we normally plan on 5% of the tankered load as additional burn, per hour...that adds up. For us to tanker just 20,000 lbs, we'd use up half of that on a ten hour leg, meaning the tankered fuel just cost us twice as much...better be a substantial price difference to justify that.
On a short leg, tankering really depends on what we need to meet our landing weights...our landing weight is 200,000 lbs less than our takeoff weight...so generally tankering fuel isn't an option in the first place for us for a short trip.
Then, looking at the requirements at the landing field, if we need to increase our brake setting and experience higher brake wear as a result, we may easily have eaten the cost of the tankered fuel or any savings thereof, before we even leave the runway. Add to that increased turn around times due to hotter brakes due to the heavier landing weight (depending on runway, of course)...we may have just shot ourselves in the foot by losing time that could have been spent flying revenue freight, instead of letting our brakes cool.
It's a tradeoff, but I don't think it's nearly so simple as saying a short leg justifies tankering the load.