Getting the last value of a rolling 12

[Victormar]

Hello, community,

I am reporting on energy consumption, and I wanna assess the deviation of its consumption based on a rolling 12 months value, so I will always assess the last value for this rolling 12. 

I know how to calculate, but I don't know how to do the complete process, from calculating the R12 to getting the deviation.

I have energy consumption for every day and it's date (although some days it could be blank), and the deviation would be this value minus the contracted energy consumption, divided by this same value (which I also have in a column). 

I am not sure if it's better to try to create a column for the R12, and then get the latest value based on the date, or if I should do a measure straight. 

Thanks in advance for any kind of help 🙂