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(rect[0], rect[1]) yes those are exactly upper left and,.. , (rect[2], rect[3]) are the lower left corners..

you can just do the following

xFaceCenter=(rect[2]-rect[0])/2
yFaceCenter=(rect[3]-rect[1])/2

and now you have the height and width of the image frame lets say h,w then you can easily use

dy=(h/2)-yFaceCenter
dx=(w/2)-xFaceCenter

So you now just have to make these dy and dx values to be zero by changing the horizontal and vertical servo angle accordingly