  The Soft X-ray Spectrometer (SXS) on board \textit{Hitomi}, with the
unprecedented resolving power of R$\sim$1250, allowed the detection of members
of the Fe XXV \ka\ complex emission spectra from the center of the Perseus
Cluster. In this paper, we introduce a novel method of measuring the column
density using the optically thin (Case A) to optically thick (Case B)
transition for one- and two-electron systems. We compare the Fe XXV K$\alpha$
line ratios computed with CLOUDY with that from the \textit{Hitomi}
observations in the outer region of the Perseus core using collision strengths
from different atomic datasets, and obtain good agreement. We also show the
effect of turbulence on Fe XXV K$\alpha$ line ratios and interplay between
column density and metallicity. Besides, we discuss the atomic number
dependence of transition probabilities for allowed and non-allowed transitions,
which causes the highly charged He-like systems, such as Fe XXV, to behave
fundamentally differently from He I.
