Set worked areas for each of your fields to increase accuracy in calculating input quantities, yield rates, etc. Worked areas can also streamline job creation and will be used rather than the mapped area in:
The area used in the field list and view
Select the field in the field list or on the map
Head to 'Worked Area', click 'EDIT'
Adjust the given (mapped) area to the worked area of the field. Use the arrows, or type in a value
Remember to click 'SAVE'
Select the field by tapping on it in the map
Tap on 'Worked Area'
Adjust the given (mapped) area to the worked area of the field.
Tap 'SAVE' in the top right