If you care about gear ratios, you also want to consider the specific choice of teeth. 52-42 works well with something like 14-16-18-21-24-28 in back (been there ... done that), just as 52-40 worked well with a 14-17-20-24-28 5-speed.

If it were mine, I would seriously consider 50-40 / 14-16-18-21-24-28 or what I used on my Bianchi, 50-42 / 14-16-18-20-23-26 (SunTour standard/wide width), which works superbly with a Campag. NR. I am currently running 46-38 / 13-15-17-19-22-25 (Regina America) on the 1959 Capo, which has a brand-correct but anachronistic Campag. 980 derailleur. I have even run a triple half-step-plus-granny with a short-cage SunTour cyclone 48-45-34/13-15-17-19-21-23, which had a full 40 to 100 gear-inch range.

One secret to obtaining lower gears on older equipment is to ask yourself whether your really need a high top gear. For me, anything in the mid-90 gear-inches is perfectly adequate. Choose a smaller outer chainring and work your way down from there.