That is the great thing about an FMC. I put in a cost index then check the progress page. Compare the arrival time versus fuel at destination and make a decision. Do I like what I see - leave it - don't like what I see - change it back.
Usually find in a 767 that a cost index of 60-80 gives a good medium between flight time and fuel usage. A cost index of 250 can be a pain, watching the speed with one hand on the thrust all night. Makes a long night!
69 was always a good number
