Project an object from a perspective camera at same screen position but different Y world position

I have a game object at let's say Position (1.0, 2.0, 1.5). I have a perspective camera. I want my game object to be at the same place on the screen position but for a Y value of 0 (so that it is perfectly on a plane). Since my camera is at an angle of 50 degrees, that means that for it to work, the Z position must be greater than 1.5 to compensate for my y=0. How can I calculate this ?