Update previous record on insert

Hi all,

 I have  a table with 5 fields......3 fields are important.

opening reading , close reading and total usuage

the table records will be displayed like this.

opening reading                close reading         total usuage

0.0000 (default)                 10.0000            10.0000

10.0000                       20.0000      10.0000

20.0000                   45.0000              25.0000

Here, based on the close reading......it should changed to opening reading based on previous records..

every record while creating for a new record based on previous record has to come. how to do? that?

I have done some changes in modified field,update and insert method...from reference from  

http://stackoverflow.com/questions/5864634/can-a-field-be-calculated-based-off-another-field

I am unable to update the opening reading from the previous record. 

Have a nice day.