How to calculate overscan…

1. By a new Focal Length:

Original Focal Lenght / New Focal Lenght = ratio
Original Resolution * ratio = New Resolution

2. By a new width:

Original Width / New Width = ratio
Original Focal Length * ratio = New Focal Length

3. By percentage:

Original Focal Length / Percentage = New Focal Length
Original Resolution * Percentage  = New Resolution

ie.-

Original Resolution: 1920 * 1080
Original Focal Length: 35 mm

New Focal Length : 20 mm

1.

35 / 20 = 1.75
1920  * 1.75  = 3360

1080 * 1.75 = 1890

2.

1920 / 3360 = 0.57142857
35 * 0.57142857 = 20

3.

35 / 1.75 (75% more) = 20
1920 * 1.75 = 3360

1080 * 1.75 = 1890