trying to determine scope's focal length, C9.25

I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

Thanks,
Partridgefamily