Gents, 5% per hour is way to high it worked for old low to medium bypass ratio engines with 60's and 70's technology airfoils, but the modern aircraft is way more efficient than that.
Unfortunately, I don't have any numbers for the B737NG type aircraft and no ability to access flight planning data for the type to run the flight plan data points to give you the fuel carriage penalty for this type. Its an interesting exercise and with good flight planning software takes about two hours to get enough data points to calculate the percentage to carry and develop a mathematical representation of the data in the form of an equation.
The difficulty is that the fuel carriage penalty is not linear over time, in fact some are quadratic or cubic functions and using a rule of thumb is if little value for this type of exercise.
The key elemts in the equation are the price at departure point, the price at destination, the stage length of the trip from departure to destination AND the stage length of the onward trip from the first destination. Complex stuff for the average line pilot and really can't be effectively done using a calculator.
The latest state of the art flight planning software available today generally all have "Tankering Modules". This takes all the above information and provides you with a tankering fuel amount and the monetary savings this will generate.
Lots of great tools out there so no need to rely on a rule of thumb.
Richard