Orp Tolip, You are confusing LRC with MRC (Maximum Range Cruise speed). MRC does what it says on the tin. LRC, as stated above by FullWings is a cruise speed that sacrifies 1% of range capability for a significant increase in speed.
On an FMC equiped aircraft ECON speed takes, amongst other parameters, head & tailwind into account. In still air, ECON speed is related to the Cost Index, CI, which compares fuel costs to fixed costs. Many pilots call the CI a 'fuel cost index', but it is simply a 'cost index'. For any aircraft there is a CI which would equate to LRC in still air.
In a headwind ECON is faster, and in a tailwind ECON is slower.
LRC is a rough and ready number to use as a trade off between fuel and range (range at LRC is 1% less than range at MRC). CI is a more scientific way, with the head/tail wind input being the icing on the cake. However, to many airlines it has become apparent that calculating the fixed costs is very difficult to quantify, so they use a single CI that seems to be 'about right' for their operation. Even if the CI is not calculated accurately, the benefit of the head/tail wind input means that using ECON may still be better than plain old LRC.
CI 0 = MRC
CI X=LRC (in still air)(most AFMs have a graph to show the CI equivalent to LRC)
CI 99 = free fuel and high speed (could be 999 or some other high number depending on the FMC)
I’m not aware of any rules of thumb though to answer TheMullEye’s original question.