Calculating operating costs

I was recently asked the following question: how do we work out the operating costs per surgery per day? My answer The calculation is:

1. assume the practice is open and staffed but there are no fee-earners and no patients

2. costs DO NOT include lab and materials or associate/hygiene wages because no patients turned up

3. but costs are everything else needed to run the building, including staff wages

Calculate that figure and divide by number of full-time dental surgery days per annum.