I have found 17T x 41T for stock circumference rear tires and 16T x 41T for the 200mm x 16s I run, to be fairly close to ideal (YMMV) for the up/down/flatland terrain here in the PNW . If I lived in Flatbush, Iowa or Illinois, 18 x 42 would probably be fine. My point is by losing or gaining a tooth on the rear sprocket, you gain or lose roughly a half value of final ratio, compared to changing the primary sprocket up or down a tooth.
