On the 737 the FMC increases the mach number by M.01 per 100 kts of headwind and decreases it by M.06 per 100 kts of tailwind.
One problem of using a CI to determine your cruise speed though is the reduction in speed with a reduction in cruising altitude.i.e. the lower you go the slower you go.
If you are trying to stay out of a strong headwind you can loose a lot of the advantage because the speed drops off considerably.