Additional Accessorial Charge applied based on existing accessorial charge on shipment

Need ideas.   I want to apply a 50 % accessorial cost based on the original accessorial cost at time of rating.   Fuel SurCharge.   We are currently applying FSC using a rate factor and rule to determine the weekly FSC, now I also want to be able to add an additional cost based on current FSC of 50 percent.   

Any ideas of simply applying 50% additional cost to existing accessorial cost without creating another rate factor or rate factor rule etc?

Thank you,

Teresa