Just for visual inspection; checked energy conservational viewpoint.

In terms of REFL power, the brighter is the better. Even while slight reduction of the REFL power, the arm transmission, PRC-inside carrier power, and AS contrast seem tolerable. But, if the REFL power reduced further, drastical reduction of the arm transmission, PRC-inside carrier power, and AS contrast quality simultaneously happens.

For example, if just only looking at the REFL power, and if it reduces like the one indicated as "2" in the attached figure, one may think it would be obvious that the contrast of the interferometer would get worse, but the fact is not so simple; seemingly not so "entangled". The phenomenon is not so linear. Where the energy went...? As if there would be two different causes of the REFL reduction. One cause worsen of the contrast, while the other would not do so.