An automated spectral fitting algorithm (named Thermo NO-LIF) designed for the extraction of temperature information from experimental NO-LIF spectra is presented and analysed. With the aid of the Thermo NO-LIF, the high-accuracy (0.5%) of multi-line NO-LIF thermometry are demonstrated in the burnt-gases region of a near-adiabatic Bunsen-type premixed flame. The results are compared with the temperature measured using N2 spontaneous Raman scattering (SRS). The current study also applies and analyses the algorithm and the thermometry technique in a premixed flat-flame under various sub-atmospheric pressures. The results suggest that choosing a long excitation scan range with as few overlapping lines as possible can maximise the performance of the multi-line NO-LIF thermometry. The uncertainty arising from the overlapping lines is especially crucial at atmospheric pressure. Additionally, the current study compares the performance of multi-line NO-LIF thermometry using narrowband and broadband lasers. Thermo NO-LIF is available to anyone interested in applying multi-line NO-LIF thermometry.