Accurate formula to measure display GAMMA !

Dear ALL,

I am in process of measuring GAMMA of my display device, and wanted
some advice for the same.  I refer
http://www.brucelindbloom.com/index.html?Eqn_BestGamma.html
to calculate the gamma value, but not getting the correct GAMMA value
as expected.

Steps, i used to measure GAMMA:

a) Measured Luminance  for various grayscale using  a calibrator.
GrayScale values (val, val, val)
where val range from 1 to 254

b) Normalized grayscale array by dividing each value by 255

c) Normalized luminance array by dividing each value by highest value
of luminance ( 197 in my case)

d) Used formula mentioned in
http://www.brucelindbloom.com/index.html?Eqn_BestGamma.html
to compute GAMMA

Also plotted graph on excel for those values. The graph is a proper
curve as expected. BUT, the gamma comes out to be 2.01 for a display
device which has a GAMMA of 2.2. I verfied the GAMMA of device by
measuring it with a third party application which gives GAMMA as 2.21.

Also i tested with other gamma values, my application could not give
correct GAMMA values.

What can be the issue ?
Is the formula accurate mentioned in this site ?
Can you please suggest some alternative formulae?

Please comment.

Thanks,
Rodney