Calculating a template matching similarity score?

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minMaxLoc is doing everything for you. It returns the min value, the max value, their position. If you call this function on the result of matchTemplate, you will have the best position of the patch (in maxLoc), and the similarity score (in maxValue). If you used 'CV_TM_SQDIFF', you have to get the min instead of max, but it's done with the same function.

I hope it's clear...

As far as I understand you would like to get one similarity value for a whole template matching instead of the array of individual pixel matches. However the main idea of template matching is to slide a small (relative to the image) template through the image and find the highest matching area. See the template matching tutorial here. The resulting image gives a detailed map on how well the template matched in each location.

The following set of images show how well the template matches on the image with different matching methods:

Back to your original question if you are not interested in the exact location of the match just the highest/lowest match value - because you want to find the images on which the template is certainly there - then you can use the minMaxLoc function and compare the maximal/minimal value with a predefined threshold.

It seems you are using the documentation sample normalize in this samples makes the matching score always equal to one so comment this line and calculate the confidence here it's the code

int match_method = TM_CCOEFF_NORMED;
bool use_mask = false;
Mat mask;
//mask = imread("c:\\temp\tempmask.png", IMREAD_COLOR);
bool method_accepts_mask = (TM_SQDIFF == match_method || match_method == TM_CCORR_NORMED);
if (use_mask && method_accepts_mask)
{
    matchTemplate(img, templ, result, match_method, mask);
}
else
{
    matchTemplate(img, templ, result, match_method);
}
//normalize(result, result, 0, 1, NORM_MINMAX, -1, Mat());//new added check
double minVal; double maxVal; Point minLoc; Point maxLoc;
Point matchLoc;
minMaxLoc(result, &minVal, &maxVal, &minLoc, &maxLoc, Mat());
double dConfidence;
if (match_method == TM_SQDIFF || match_method == TM_SQDIFF_NORMED)
{
    matchLoc = minLoc;
    dConfidence = 1 - minVal;
}
else
{
    matchLoc = maxLoc;
    dConfidence = maxVal;
}