My rule of thumb for the altitude at which you should expect to see the threshold is RVR/3 + 50'....i.e. 600 meters vis means you should see the rwy at 250' (obviously we are talking about fog with NSC).
By this reasoning, to calculate the vis from an altitude at which you first see the rwy, simply subtract 50' and multiply by 3. I.e. see rwy at 250' = 200 x 3 = 600m vis.
Very TLAR but found it's usually close!