My Gavia is a 49 (maybe 48, can't remember) and according to the numbers approach I should be riding a 51. By this logic they come up slightly large and you might get away with it, any chance you can try before buying?
Think it will definitely depend on your proportions overall, arm length, torso, etc. My road bike is a 56 and I'm 6 foot, but as most of that is in my legs (about 34 inches), I was actually debating a 54 as the reach on the 56 is quite long...Try before you buy is indeed your best bet!
You really need to try the frame out - I'm 5' 10" and a bit and a 54cm fits me. But there are so many variables like handlebar stem length and layback on the seatpost - although they can be changed at a later date.