That's a good question. Ideally, the focal point would be inside the feed, a small distance from the plastic cap. Trouble is, it is really hard to say exactly where because this depends on the feed design. No question that it is not at the plastic though.
If you want to find the focal point of the dish for some experiments you could do so using optics. One way is to cover the dish surface with reflecting foil (carefully formed to sit right on the surface of the dish) then aim it at the sun. Next take a small piece of something black, like paper or whatever and move it around where you think the focal point is and observe the focused spot of light get larger and smaller. Where smallest, that's your focal point. I guess you have to be fussy to make the foil smooth.