score:9
According to Google Street View, Santa Maria sopra Minerva is actually right behind the Pantheon (with a small square in between) and the street level for both buildings seems to be quite similar. Wikipedia has an image by User Peter1936F that shows some more flood markers. I have marked the flood markers from 1557 and 1870 in yellow and red, respectively. Judging again from google street view, the height difference between the two markers seems to be about one and a half meter.
Why 1870? Because for the 1870 flood there are actually photographs of the flooded Pantheon, e.g. here.