Calculating Expressions based on other Fields.

Hi,
I have to calculate Values Based on Expression given for Individual Line.
For Example:
In Sales Order Screen Line Details I have 3 Items.

Item 1 Qty Value Expression will be-- Item 2 Qty  + Item 3 Qty + 5 ;
If I change Item 2 Qty, Then Automatic Item1 Qty will get default based on Item 1 Expression..
 

Expression will be Dynamic.

Like this Concept how can I create Expression and Default the Calculations. 

Thanks,
Giri.