On the PT745 Millennium Pro (and I assume pretty much the whole Millennium Pro Series), the plastic/composite button on the larger end of the two-piece spring assembly goes just barely through the hole in the front of the slide. The metal flat of the spring assembly (smaller spring end) contacts the vertical flat on the underside of the barrel at the very front of the chamber area of the barrel.
Getting this positioning correct seems to be hit and miss for me. If it's not just right, the slide will not cycle fully to the rear and then I have difficulty loading my first round from the magazine. Functioning, while firing - thus far, has been fine. My problems have all stemmed from assembly and the fact that the slide will not go fully rearward after reassembly. I just check and recheck until I have it fixed.