ZSSS - ZLIC as an example, RI currently 30, and I see the system tells me the route is created 1981-09-23, and last flown on previous game day (1982-08-27).
Since route has been active already so long the RI should be near 100 - unless that route has been inactive (not being flown) for several months in between (which I can't tell).
Have to keep an eye for some game months if there are changes.