How to calculate confidence of Flann

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I 'm using the following sample [Features2D + Homography to find a known object] (http://docs.opencv.org/2.4/doc/tutori...) to find rotated and scaled object .

i want to find the confidence of the detected object in the scene how to convert the good matches in the scene to confidence (0 to 1) , also what is the threshold to accept or refuse this match

thanks for help