First thing, I was wrong in airbus 320 approximate CI. It is 45, not 40.
LRC is one thing and MRC is quite another
MRC: CI=0
LRC: go to the performance tables to find cruise MN or just consider inserting CI 45 because it will give, approximately, LRC.
If I recall correctly, LRC whas that cruising speed such that it gives a 99% of the MRC, which gives a considerable time saving (in the order of 10% or more, maybe?). It makes it more confortable to fly, too.
As a rule, it is not a big deal if you are not flying exactly at LRC, or slightly faster. So I guess I would just insert 45 in the CI field if i wanted to fly at LRC. If I want to be very very exact, then I will reach the FCOM 3. Even though, we can't select mach numbers to the thousandths.
Anyway, it still puzzles me that fact, ECON speed being slower than LRC. So if we divert to the alternate after a pull up in destination airport, we have to accelerate to LRC??
Another thought. Shouldn't the CI be different from A to B than from B to A if fuel prices are different in A and B?