Originally Posted by
bidaci
Quoted from Sheldon "Adding or subtracting a link in the chain will move the axle 1/2". Changing either sprocket size by one tooth is the equivalent of moving the axle 1/8" (4 mm)."
1/8" = 3.175 mm. A lot closer to 3 than 4.
I filed out my dropouts enough to allow about 1/8" of adjustment. When the chain stretches that much I put on a new one.