What is the underlying formula used to calculate the default range in Renko charts for a script, given that it doesn't align with the conventional ATR14 method?

The default brick size in Renko charts is calculated using the Caveman algorithm. This algorithm uses 300 aggregated bars of data on the chart (i.e. about a year for daily charts and about 5 hours on a minute chart) and comes up with a reasonable default value. So, the default range will vary depending on the asset (instrument/scrip) viewed.