Meatlover, converting Vis, does not depend on the reported vis but on the Required RVR . The required RVR of the approach plate ( in your jeppesen) must not be lower than 800m.
If your approch plate requires an RVR of 700 m, you cannot convert anything. If the tower gives you a vis of 1000m you cannot shoot the approach. Because you need an RVR and you are only allowed to convert when the REQUIRED is not below 800 . So in my exemple any reported vis is useless from a legal point of view, whether the vis is higher or lower .
2nd exemple: if your approach plate shows an RVR of 900 m you can convert the reported vis by tower. If they give you 450 m , you might be fine by night in HIALS and runway lights equipped airport.
Eu ops reference: "An operator must ensure that a meteorological visibility to RVR/CMV conversion is not used for takeoff, for cal-culating any other required RVR minimum less than 800 m, or when reported RVR is available"