The time-honoured 737 Classic 'rule-of-thumb' is 4% per hour. If the computed flight time x 4% is less than the price differential you tank if you can. Take a 3 hour flight: it will cost you 12% of the fuel you tank. If the price at the other end is 15% more than departure, you win. This 'magic' figure is well-proven and takes into account extra burn and lower levels. You need to decide the differential gain at which it becomes non-economical to tank (but as mentioned before, sometimes other factors like turn-round times come into play).
It is fairly easy to write an Excel sheet to do the work for you, but things to remember:
Do not tanker a lot more than you need to return!
Take into account minimum uplifts if any at destinations - some refuellers have a minimum amount rule, so you will finish up paying heavily for 400kg say. Better sometimes to 'undertank'.
Publish the recommended figure on the pilot's plan OR let them work it out from accurate and up-to-date price differentials. They will decide what is 'practical' and should have a feel for how much they need for the return.
If the return is another crew, give the outbound crew the return tank requirement. We've done it for ages this way, and I'm sure it has worked.