Tim,
I would double check the focal length on the Moon or the Sun. Even though the hill was a couple of miles away it wasn't at infinity and that will make a difference in the focal length at infinity. I'm a little surprised you came up with 86" vs something closer to, or actually greater then 90" since the lens is suppose to by f/15. Lens maker are usually really good at hitting radii or the performance of the lens will greatly surface. Usually the focal length is off by no more then 1% or less. Also since the hill wasn't at infinity, the focal length you measured is longer then what it will be when the lens is focused on an object that is really at infinity. Are you sure you had crown element facing skywards ?
I calculated that if the lens had 90" Fl at infinity and the hill was really 1.5 miles away, the focal plane should be at 90.9". ( Correction, it should be 90.085", Thanks Mike)
- Dave