From simple formula to MATLAB for image analysis: is this correct?

To measure the intensity of coloration in a histology protocol, some people (Choudhury et al, 2010) proposed this formula to give a number describing the general intensity. I'm trying to transfer that said formula in a MATLAB program for a confocal microscopy protocol.

The formula: http://imgur.com/8QdGay9

And here is my code

%The formula this program emulates is ATM = 1/255 * n^-1 * [sigma] t from 1
%to 255 of [sigma] pixel 1 to n only if the intensity is higher than the
%treshold (t) and if so, the value is 1, if not 0.

%This is the transformation to grayscale 255
imGS = rgb2gray(imRAW);

%Determination of n, the number of pixel in the image
n = numel (imGS);

%Creating a fix v;alue of imGS
imGSF = imGS;

%Coding the sum
sum2 = 0;
for t = 1:255;
    Z = zeros(size(imGS));  % Make another array to fill up...
    for ii = 1:numel(imGS);
        if imGS(ii)>t;
            Z(ii) = 1;
        else
            Z(ii) = 0;
        end
    end

      sum1 = sum(sum(Z));
      sum2 = sum1 + sum2;
      imGS = imGSF;
  end

%formula
ATM = 1/255 * (n^-1) * sum2

It seems to be working, at first glance, but being a brand new user, I wanted to have your opinion on this. Any general tip is also welcome. Sorry, if this post is not conform to your expectations.