Yes, I know that it was possible to space back even one unit (and find your place with that little wire you popped up ) but to use the technique that worked on the Selectric you would have had to memorized the unit spacing of each letter, add all the weightings up and divide by two. Actually what I used to do was take a separate piece of paper, type out what was to be centered and then hold it over the new good page that had a small pencil mark in the center. Usually we didn't have to do that because we used the Executive only for letters and we already had letterhead at the top of the page. Billing and file copies were done on other typewriters - for reasons that were obvious to us but apparently not obvious to some journalists.
Yep. I would type it on scrap. Measure it. Divide by two, then backspace from the center by that number.
Now the young'uns know why we are such good typists. Every thing had to be typed twice!