We can “build” a bike from the pads down if we know this metric. How do we know your BB to Pad X and Y distances? You measure it off your existing bike if you like your position; you go to someone you trust to fit you if you aren’t confident in your position and then he or she measures it.

The blue diamonds in the chart are the Pad height (Pad Y) and the Pad length (Pad X), of 3 dozen Slowtwitchers who’ve measured their bikes and reported the numbers. As you might guess, as taller riders have greater pad heights, and these taller riders' pads are also pushed further in front of the bottom bracket. Therefore, if you plotted the Pad X and Y of 5 riders, each 2 inches taller than the next, and you plotted the intersections of their Pad X and Y on a Cartesian graph and drew a line through those plotted points you’d get a inclined slope and that’s more or less what we’ve done here.