9.G thats correct!
see 6 lights = 90 m from flight deck - think about that you see the lights in the slant-range only.
this gives the required actual 125m
True, but this gives you a 90 meter segment. We want to replace a 125m lateral segment as RVR was 125m too.
To have 125m lateral you need to see more than 8 lights to my opinion. (15 meters spaced) Your slant visual segment will be in that case more than 125m, but no one cares about that one as you need a lateral segment of 125m (just like the RVR is)!