120 should give you an approximation of M.84.
18 should give you an approximation of MRC.
We presently use 75, this is reviewed every quarter and changed accordingly, we dont use route specific values. If we want to avail of cheap homebase fuel, we can use 450, this gives an approximation of M86.
I would presume that slowing down to CI=18 cost more in flight time than the fuel saving gained, so it makes sense to return to a higher CI value.
Mutt