You’re trying to keep the face centered?
Let’s say ServoH sweeps from left to right and ServoV sweeps from down to up. I expect OpenCV will give you the location of the bounding box. Average the X coordinates to get the horizontal center of the bounding box. Average the Y coordinates to get the vertical center of the bounding box. If the center of the bounding box is to the left of the center of the screen, decrease the angle on ServoH. If the center of the bounding box is to the right of the center of the screen, increase the angle on ServoH. Similarly for the vertical axis.
If you want to get fancy, use the horizontal and vertical angles of view of the camera to determine how many degrees away from center the bounding box is. Use that number of degrees to adjust the angles of the two servos. For example, if the horizontal field of view ia 100 degrees and the bounding box is to the right of the center by 10% of the window width (10 degrees) then add 10 to the ServoH position.