Hi Guys: Steve that works but not for refractive optics of course. Here are 3 simply ways. They are not the only ways but they are a fair start.

Negative refractive element: Place (1/2") aperture stop against lens surface and pass sunlight through lens, measure backfocus to point that spot is 1" in diameter. This is going to be pretty close to negative Fl.. Take another measurement where spot is 2" in diameter and divide by 3 for refinement on the answer.

Negative convex reflector element: Do the same except reflect sunlight through the mask and back onto a scaled target. A square mask may be easier to use.

Negative concave refector. Using any light source at any distance greater than Rc pass light through hole in paper and allow returning light to come to focus the image of the hole on the paper. This is the Radius of Curvature which is twice the focal length.

Even though the sun is not a point source the spread is less than the error in reading saggita is likely to be.

Cheers, -Rick