The RVR needs to be reported, on the ATIS and from ATC. If you don't have an RVR reading, you have to use the min Viz figures. So if you have RVR use 550m, if the reading is not available use 750m. Works well around the world.
There use to be charts to factor Viz to work out the RVR, but most are not allowed to use those anymore.
The RVR transducers are spaced at touchdown, midpoint and roll out ends of the runway.
Not sure if this answers what you were asking or not.