From practical experience even a Cost Index of more than 10 sometimes give a lower speed than LRC if you have a strong tailwind (FMC deems it more economical to hang around in the tailwind for a little longer and giving us a free ride).
Normally about 30 gives you LRC but quite often you can see that even a ci of 40 or so will be slower than LRC.
Is there a way of fooling the FMC during the set-up to determine your minimum drag Mach no. in the cruise?
I was thinking that putting in you FL and zero wind, and ci 0 might do it. Thoughts?