calculation based on a previous record

[KTJ_SBS]

Hi,

I have a table full of meter readings. I want to have a field called consumption which looks at the reading just entered for that month and then subtracts the previous months reading for that meter which leaves the consumption.

Can I get a formula that can work this out automatically?

 

[KTJ_SBS]

image below

 

[KTJ_SBS]

sorry i need another post to show image

 

[KTJ_SBS]