My OME springs came in yesterday and now I see where the issues are in the calcs.
The free height spec is correct, but as I though, the 8.9 coils includes the closed & ground end which is not considered part of the "active coils" for the estimates. It also throws off the adjusted free height estimate a bit. Another issue I found is that the top coil is actually closed as well (last coil flattended). Cutting this section off doesn't change the spring rate much.
With 1/2 coil cut off i'll be ~14.25" free height
With 3/4 coil cut off i'll be ~13.75" free height
With 1 coil cut off i'll be ~13.38" free height
I think i have an idea...need to go measure my XK again