At least within the middle-to-large frame sizes, most of today's endurance and race bikes are featuring geometry very near 73.5-degrees seattube and 72.5-degrees headtube, for best all-around handling and performance. I suspect the Specialized Roubaix frames are no exception.
This geometry tolerates a wide range of stem lengths well and works well with today's often-taller headtubes.
There are many vintage bikes with similar geometry, the golden key is to have ridden bikes with known geometry, and so then having some preference for which direction to change angles from there.

Note that frame angles affect fit as much as they affect handling, and that the two are intertwined:

--Seattube angle affects the frame's (forward) reach dimension and thus affects the stem and toptube length that will be needed to position the handlebars optimally (fore and aft).
--Headtube angle affects steering response, so a steeper headtube angle tends to work better with a longer stem length.

0.9cm per degree seems like a good rough estimate for adjusting either/both of the above.^^^

Note also that certain brands of vintage bikes made huge-swing changes to their familiar bike's geometry from one year to another, as when Peugeot changed their nice, calm PX10 geometry from ~72-degrees to ~75-degrees around 1973. So know what you are buying or you may go through the sort of countless frame-change experiments that many of us have endured (and made a habitual routine of).