Updating a Variable from a Field

I'm creating a calculated field to calculate freight rates based on a fuel surcharge inside a module I call "Freight Rates".  Essentially, I need a place that an employee can enter the price of fuel once and it will automatically calculate all of the rates in the "Freight Rate" module.  Any suggestions on where I can place that field?  The only thing I can think of is placing the field on the record and use a mass update to change them all, but I'm guessing we will end up with 10,000 records that need updated weekly.  Thanks in advance for any suggestions.