I did some homework on my own question.
Assuming (but is that correct?) that the X-dim shown in the performance report (19) is meant as the distance between the engine crankshaft and the bottom of the hull, I checked the Mercury drawings of Bravo X.
By the way, I coulnd't find the Bravo XR drawings, but I guess there's no difference in dimensions (again, is that right?).
Now, the distance between the engine and the prop shafts according to these drawings is 21" 13/16.
Therefore, unless I'm missing something, the prop shaft should be (21 13/16 - 19) = 2" 13/16 below the hull bottom.
I know there are many experts around here who will smile at such a basic question, but I'd really appreciate if any of them could confirm and/or correct my figures as appropriate!

Thanks in advance!