Calculate tilt stiffness

Folks,

See attached. I wish to calculate the tilt stiffness as a function of some linear stiffness.

My attempt is on the RHS of FBD and there is somebody elses attempt on the LHS. I am not sure if it is right.
I have 2 queries

1) My attempt on the RHS I wrote the expression of f in terms of linear stiffness (data of which I have lets say 100N/mm). How do I get the actual force? Ie, I am not sure with moment arm to multiply this stiffness value by?

2) How to write the expression as a function of the angle?

Thanks

NOte this has been posted in
Calculate tilt stiffness which is gone off the first page of threads and no response.

Regards