I need to calculate a performance percentage based on the lowest cost in history and the current cost. Example: For all Fridays, I have a lowest cost of $0.52. I want to convert this into a percentage. So if we go below that - maybe to $0.48 that would be a good performance number. As .48 is a 8% efficiency, I want this to produce a # like 90%. And as the cost goes down, the performance percent will go up.
I am using MS Access or Excel to create this calculation.