The equations are derived by third order polynomial curve fitting of the data given in SP-16 table in Indian code.
For Fe 250:
y = 10802254950x3 - 47811496.7x2 + 74481.40335x + 205.8829164
For Fe 415:
y = 8634745755x3 - 84776621.6x2 + 284712.3995x + 29.42076793
For Fe 500:
y = 7949113254x3 - 89302413.2x2 + 343343.7433x - 20.40632679
