Actually, this is a mathematical question and belongs to math.se.
Moving the camera "down" would be the same as moving the object(s) up. I'll move the object up for illustrative purposes.
For the calculation, you need the focal length and the sensor height of the camera. They are marked lime in the image. You can get that information from the camera. I've set the sensor measurement to vertical. If it is set to horizontal, you would have to calculate the vertical sensor height from the horizontal sensor height and the dimensions.
verticalSensor = horizontalSensor / horizontalDimension * verticalDimension
Next you need the distance from the camera to the object. It is important that the distance vector is an extension of
(0, 0, 1) from the camera. The vertical movement is labeled as Object Shift. This vector must be an extension of
(0, 1, 0) in camera space for your case. (If the first position of the object is not on the vertically centered horizontal axis of the cameras view space, you have to adjust the calculations.)
With the rule of proportion, we can get the relation between these four values.
sensorSpaceMovement / focalLength = objectShift / objectDistance
sensorSpaceMovement = objectShift / objectDistance * focalLength
To get the sensorSpaceMovement (the shift on the sensor) in relation to it's size, we divide by half the sensor's height. (Note, that the lime arrow is half the image's height in the illustration.)
sensorSpaceMovementRelative = objectShift / objectDistance * focalLength / sensorHeight
The objects shift is now relativ to the sensor size and will have a value in [0.0, 1.0] if it doesn't leave the field of view.
To get this value in pixels, multiply it with the number of vertical pixel rows: dimensionY. This is the height of your rendered image.
pixelShift = objectShift / objectDistance * focalLength / sensorHeight * dimensionY
If the relation is rotated in 2D or 3D space, you'll have to use a little more math. The object shift has to be measured relative to the camera's XY plane and the objects distance parallel to the cameras (0, 0, 1) view vector.