Are you referring to the commanders assessment of the 90m visual segment?
What it states is that if the point A (to use your reference) is less than the 150m or 125m (if authorised) RVR then the commander can look at the centreline lights which have 15m spacing. If he can see 6 lights then he can see a 90m visual segment and he can accept a takeoff clearance.
However is the RVR is at or above 150(125)m and you cannot see the 6 lights you must reject the takeoff clearance as you need confirmation with the visual segment- the important thing here is what you can actually see.
Another point to consider is airfields with displaced thresholds but departure points from full length of runway. It is possible that the centreline spacing before the threshold is 30m spacing and as far as I know you cannot count 3 of these as an equivalent to 6 centrelines so you must plan to depart from the displaced threshold point.