Fig 14 in Jerkstrand+2017 plots various quantities related to [Ca II] 7291,7323 and Ca II triplet formation. The calculations use T, n_CaII, n_e, and t as parameters, and solve the NLTE populations and line strengths. t was fixed at 300d and the others allowed to vary.
First, note that radiate transfer effects can influence the Ca II lines in many instances, this is not considered in this grid which only considers thermal collisional processes, and spontaneous radiative decay (including Sobolev optical depths).
One important question is the interpretation of Ca II triplet/[Ca II] doublet ratios exceeding 1 by large margins. If collisional processes are behind this, Fig 14 shows that high electron densities ne >~ 1E8 cm-3 are needed.
The formation regimes and general ratio dependencies can be seen in the figures.