The analysis of the data, exploring the other studies conducted using a Sky Quality Meter (SQM) and naked eye observations all point to a given light level (magnitudes per square arcsecond - MPAS) which can be perceived and matches the criteria set for Subh al-Sadiq (dawn) in Islam.
This study found it to be 19.5 MPAS. This level of light (19.5 MPAS) on the scale shown below is still in the band of darkness where stars can be seen.
The question then remains; is this a universal light level reading which can be applied to all locations across the world for Subh al-Sadiq (dawn) or is a product of analysis in a given location (latitude) ? The basic answer is that without further research in multiple locations (latitudes) there is no way of knowing and we have reservation about making the same universal statements (like dawn occurs at 18°) which causing difficulty and confusion.
It may be that the light level reading for Subh al-Sadiq (dawn) are relative and not absolute. It was the case that this study was still able to conducted naked eye observations for Subh al-Sadiq (dawn) for days where the MPAS reading was not close to or at 19.5 MPAS. Having said this, when the LOWESS function was applied there was a strong correlation at the calculated time even if the light level reading did not reach 19.5 MPAS on the given night (e.g. middle days of the lunar month).