Help needed with pivot table - need to select minimum value between 2 field counts

Hi All,

I'm trying to create a report which looks at the energy bills of a building before and after upgrade works were carried out.
I need to count the number of months of bills for the before and after works.
More often than not I have a different quantity of bills before and after so I need to pro-rata the units of electricity used based on the lower number of bills I have.

For example, I have 12 months of bills pre upgrade works and 8 months of bills post upgrade works.

I want my pivot table to be able to tell me which is the lower number of the 2 so I can pro-rata the information based on the lower number.

I need to be able to do this for both electricity and oil.

On the attachment on Sheet1 I have the data set and an attempt at a pivot table
On Sheet5 I have an idea of what information I need to present

Can someone help me out please!!