In Renko charts, what is the underlying formula for calculating the default range (as range does not match conventional ATR14) for a scrip?
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.