If it's a 4.6" lens and it's F8 then the formula is Focal Ratio = Focal Length / Diameter of Lens.
Thus, moving things around so that you have the two known values on one side of the equation you get 4.6 * 8 or 36.8".
This value is going to be fairly close to that value but may vary by about 1/8" or so from the real value. You can find the exact value by putting the objective in a tube and not attaching the back end of the tube to the tube, check the focus of the scope. Start with the tube long and cutoff the tube by 1/2" or so at a time until you have the focus where you want it.
I might note that the EPs in the 15-32mm range have a nice field stop (the ring that doesn't seem to be doing anything in the EP's barrel) and that is where the focus actually is of the objective when you have the scope focused.

• Thanks Bob ! - icstars2 - November 17, 2001 - 23:48 UTC