Writeback table - Total in real time for a numeric column

Hi !

In a writeback table, I would like to use the "Total line" option to see the total of data I have type in a numeric column. 

Ex. : We need to enter a total target of 1000 for 10 sales person. If I entered 100, 75 and 80, I want to see 255 as total. When all my 10 sales person are filled, I will kno if I put 1000 or 995 or 1005 and correct directly, before submitting data.

Thanks for your support