you usually use that calculation to make an idea of a possible RVR, but if it's not officially reported on the METAR/ATIS you cannot use it for landing decision.
I've always known that if (for CAT I ILS) you need RVR =/+ 550mt or 800mt VIS , and you're only given VIS=400mt, then you're supposed to continue the approach whilst still under the minimum, but then you must go around at the OM.
The general rule is that the most restrictive condition is the one that you must take account of.
Regarding the VOR approach, I've never known the ceiling is only for planning purpose. Anyway,why should that be? If you have BKN003 and the MDH is 420ft you will hardly be able to seethe runway land since you are at the minimum and have 120ft of clouds under you.So you cannot go lower than 420ft and should go around again...
Hope this helps your doubt
