I have created 3 fields in my invoice for my water company that is :- 

- Previous Reading
- New Reading
- Actual Reading

Where I have computed the Actual reading = New reading - Previous Reading
and New Reading can be typed in. 

I want the previous reading to be captured from the last "New reading" of the last invoice of the same customer. this way I can get Actual reading. 

I want to do this using the compute function of Odoo SAAS.

Kindly assist