How to find focal length for LNB

Hi
I need help in determining the focal length of my offset antenna.
I have an antenna where the arm on the LNB is not done correctly and is not in the centre of the focal length.
Currently, in order to watch TV somehow, I have to find the best place for the LNB by "eye" using a satmeter and a string to fitted.
I would like to do it properly, using the rods screwed to the antenna and the arm with the LNB.
Hence my question, how do I calculate the focal length, where exactly should the LNB be?
The dimensions of the dish:
89cmx81cm
The deepest spot is 6,5cm
Thank you