Find the centroid of spectra, plot and analyze!

I have few spectra from Fiber Bragg Gratings, it is text file having 3 columns (we will consider only 1st and 3rd column, which are wavelength and power values respectively). See the attached program and files I am using currently to open spectra.

I am using the equation [ Centroid = ∑(wavelength.*power) / ∑(power) ] to find centroid of spectra.

Question-1) As shown in attached image, when I plot multiple spectra, the centroid values are outside the spectra. Are those values precise?

Question-2) If not, what is the solution and is it necessary that centroid value should come inside the spectra somewhere?

Question-3) If I get the correct values of centroid for multiple files (let say I have 5 spectra and 5 centroid value), how I can make a matrix which can find the difference of location from one centroid to another, with respect to x and y axis? for instance, 2nd centroid has 2nm shift on x axis and 5 dB shift on y axis compare to 1st centroid.

Note: Attached centroid.txt file,one can run in matlab and can use attached other two text files as sample spectra.