I'd actually advice to ignore power rating *entirely*. It's calculated simply by adding up ATK, DEF and HP. And while girls with high health can sustain more damage, they don't necessarily deal more damage during their extended period of survivability. Actually, sooner or later, you will reach a point where your girls rarely die at all, which in turn means that all HP beyond the amount required to stay alive during the fight is pointless. Yet, that surplus of useless HP is calculated into the powerrating, and it makes for a rather big chunk of it as well.

If anything, I'd like the developers to overhaul powerrating, in such a manner that ATK counts double and HP counts only for about 50%. That still wouldn't say anything about a girls' actual strength (like, Anemones' main strength lays with her special attack and her DEF counting towards her counter-ability), but, at least, it'd make the figure a lot less skewed.