extract a grid value from new field

Hi all,

My question is -  I have an object say "A" where i need to select user and provide his rating through grid control. A user can have many rating. Say- user "abc" has given 5 ratings - "1,2,3,4,5" so I calculated average of all the ratings in the grid by creating a read only field in grid. This is done.

Now i want to assign this average value to some field say "newfield"(decimalt type) in an object "A". means I want to extract the value of "average" field(exist in grid) to a new field named "newfield".

Please suggest me and find the solution soon.

Thanks.