The near-infrared calcium II triplet (CaT), around 850 nm, is a key metallicity indicator for red giant stars. We present a revised [Fe/H] calibration as a function of CaT line strengths and four luminosity indicators, including the Gaia G band, together with the classical V, I, and $K_s$ bandpasses. For this purpose, we used a sample of 366 red giant stars belonging to 25 globular and open clusters, complemented by 52 extremely metal-poor field giant stars. The CaT line strengths are determined by fitting Gaussian─Lorentzian combination profiles using the PYTHON LMFIT package, which utilizes the algorithms implemented therein. The derived calibration is valid for a wide metallicity range, $-4$ dex$\lesssim \mathrm{[Fe/H]} \lesssim +0.15$ dex, and for ages older than $\sim$200 Myr. In addition, we performed a detailed assessment of how factors such as spectral resolution, spectral quality (expressed through the signal-to-noise ratio), and the algorithms used to constrain the line profiles affect the measured line strengths and the resulting metallicities.