I re-checked notes on my attempts to compute the elliot waves manually and found this excel based calculator made by me where I had attempted to predict where a particular wave would end. All of the constants seen on excel are from a book that I have.

Attached is the spreadsheet image

and these are the chart of waves that it computed

Looking at the NIFTY high low values present in spreadsheet, I presume that this is from time before I found the software tools that did the computations.

In all Elliot wave computations that I have seen thus far and from what I can recall, most important point is in determination of the Previous High and Lows. If you get that right, rest of the calculations fall in place.

I have no idea how any software determines this.