Hi Praveen,
In order to add a custom field to the EstimateLine grid, you’ll need to first add it to the EstimateLine table. You can do so by navigating to Customize > Tables / Fields, locating the EstimateLine table and selecting the Edit Fields… link.
Once you’ve added the field to the table, it will be available in the grid wizard the next time you enter Edit mode on the screen.
The logic that calculates the amount field automatically exists on the backend and cannot be modified or viewed at a screen level. You can however, create your own logic for calculating your custom field values using the Edit and Add grid actions. These are located on steps 5 and 6 respectively in the grid wizard. I hope this helps.
--Audisho