Now that's a good one !
I don't think anyone can give you
"hard numbers" or
rules of thumb for that, unless of course you find an FMC geek, or an engineer from Boeing...
Hopes this helps : On a 737-800W, U 10.6 FMC, and a cost index of let say 40 (and in ACT ECON CRZ ) in zero wind your CRZ Mach No. will be around .787-.790! In a 100 kt HW it will probably increase up to .795-.80. the most that I have seen was .805 in a 160kt HW. TAS wise the increase is close to 10 kt....
My company never uses LRC (unless we're getting low on fuel for arrival,
but to be honest it doesn't change much for the last hour of cruise).
Not exactly what you were looking for but hope it helps !