I purchased the Sigma Sport BC 16.12 STS CAD. My LBS programmed my 700x47c tire as wheel size 2268 per a CHART from Sigma Sport. The packaged instructions' CHART has 2250 for this tire size! NOW I'M CONFUSED!. Which should I use?
It's less than 1% difference, it really doesn't matter all that much. If you want really accurate, you need to do a weighted rollout test; the numbers in the instruction are just the computer maker's estimate for a typical tire, but tires of same nominal size can differ in reality. Mark spot on ground, get on bike, line up valve stem over mark, roll forward 2 or 3 revolutions, measure the distance and divide and get the mm circumference to use. Alternately if there is some road nearby with mile markers you can just ride it, note discrepancy vs. your current setting, and adjust accordingly. Or measure a ~mile route with google maps or something.