A Boeing survey showed that for the B737NG values of CI in the range of 10-30 are often used. Note that a CI=0 equals MRC. LRC is roughly around a CI of 35 on the B737NG. The actual, optimum CI can differ between airlines and routes. If you have a lot of connecting passengers, you might be better of flying at a higher CI than when you don’t have any connecting passengers on board for the same route/aircraft.
I noticed that you (
Jinkster) are using a low CI at your airline. Flying at low cost indices can result in speed instabilities. Perhaps you have experienced a lot of movement of the AT when flying at this low CI? Note that not every airline reports these problems when flying at low CIs however. Flying fixed cost indices on all routes is not always the best thing to do either.
Boeing has written a number of interesting articles on this topic. See for instance:
http://boeing.com/commercial/aeromag...7_article5.pdf