Hello Iyyappan S;
I don't know why you would wonder for so long about how a system was "derived".
As I said before, I believe a system was designed, and values selected.
The closest explanation I can find, if you examine the table I proposed earlier, and others in the Module Data manual, is that:
So this allows about 85% of the full range for the rated range values, a zone between 85% and 99% for overrange, and above 99% for overflow values.
Also, 27648 = hex(6C00) and 32512=hex(7F00); possibly the "00" ending of the hex values can be used easily as markers in their math calculations and compares. We did this alot when programming microcontrollers, in the past.
As noted, this is purely an educated guess on my part.
Hoep this helps,