Oh, I have the formula somewhere. Probably in the lens library that came with OSLO. And at least half a dozen other places...
The basic Petzval construction has two cemented elements in front, and two separate ones in the rear. The inner one of these is negative. The focal lengths of the two groups are approximately the same.
In my lens, the rear has only a single positive element, with a focal length much shorter than the front group. The mounting for the rear group also has a place for one thick-edged element, further indicating that there's something missing there. As is, the focal point of the whole lens is only a few centimeters off the back of the lens. From the 7cm diameter of the front element, assuming f:3.5 should give a focal length of around 250mm...