Los Cabos,
Your easiest solution is to go to your flight plan provider, most of them have the ability to input the departure and destination fuel prices together with the percentage of fuel required to carry that additional fuel, you will then get an economic tankered fuel quantity. Otherwise, use the fuel differential and tankering charts in the AOM and work it out.
OldeBloke, "tankering to landing weight" isnt the simple solution that it appears! You need to know the fuel prices, remembering that it costs fuel to carry fuel, you have to make sure that you have a sufficient price differential to pay for the additional fuel.
Then there is the subject of "de-icing", if you land with additional fuel, is your aircraft prone to upper wing icing?
idg, the 5% is aircraft specific and doesnt multiply by the hour. Or least it doesnt in the aircraft which we operate.
Mutt.