Looking at the Sheldon Brown gear calculator and the one found on the Wabi site show two significantly different calculations. i.e., the Wabi calc (for 700c) for a 48/18 reads to be a 69gi, but when using the Sheldon calc, and after selecting the correct tire size and crank length reads to be a 67.4gi. Which is more accurate? Which calculator do you all use? Thanks.