TMPGEnc - Clip Frame - Add pixels

[EC]

I want to know how can I save the source aspect ratio when I convert a movie in TMPGEnc. I use the Clip frame option to add pixels in the top and in the bottom. but I want to save the source aspect ratio so I need to add pixeles in the Right and in the Left.
How can I calculate the pixels in the right and in the left for save the source aspect ratio?

[EC]

Another quation: Can I convert AVI 20:9 to 4:3? How?

[the * dude]

whoo the wot the 20:9

hummm, set you output to 4:3, what matters is the way you input the avi...... in most cases its just a 1:1 ratio (just the resolution is wider - not anamorphic) so pick this as your input aspect ratio. 8)