There are 7 fields on the Quote Product form as follows :-

Price Per Unit
Volume Discount
Quantity
Amount
Manual Discount
Tax
Extended Amount

These fields are calculated as per the System Rule and the final result is displayed in the Extended Amount field.
Many users want to edit this calculation flow and create their own. We tried creating alternate fields for the same and then applied Business rules to calculate the Extended Amount but couldn't achieve the correct result as the system replaces the value in the Extended Amount once we refresh the page.

We cannot create an alternative field for Extended Amount because the value of the field is then carried forward further to the Opportunity form where Revenue and Estimated Revenue fields are generated depending on the sum of all the Product's Extended Amount field in the Product Sub Grid on the Opportunity form.