older atomic data lead to a better agreement in the Fe$^{++}$ abundances derived from all the available \[\] lines, with standard deviations of 32%/39% for the old/new atomic data (Cn 1-5), 12%/25%(M 2-4), and 23%/25% (M 2-6). We performed a similar comparison using 14 out of the 15 \[\] lines measured by @Esteban_04 in the Orion Nebula (we excluded the line at 4926 Å because it leads to much higher abundances with both sets of atomic data). We find that the new atomic data lead to a Fe$^{++}$ abundance that is 15% lower, whereas the standard deviation changes from 15% to 30%. Hence, we perform our calculations using the older set of atomic data.

We use up to 9 \[\] lines for each object, excluding those suspected to be contaminated with nearby recombination lines. For the eight PNe in the sample with no \[\] lines in their spectra, we estimate upper limits to their Fe$^{++}$ abundances using the intensities reported for $\lambda$4658 or $\lambda$4661. These lines are close in wavelength to \[\] $\lambda$4658, the brightest \[\] line for the physical conditions of our PNe. We use $\lambda$4658 for IC 4406, IC 1747, JnEr 1, M 1-42, M 3-29, and NGC 