I certainly can't speak for Damon or Slowman or others with probably a better understanding of stack and reach.
But, measuring stack and reach from the center of the bottom bracket standardizes the measurements regardless of what brand or type of bicycle you ride. The point from which stack and reach is measured is static. It is ALWAYS the bottom bracket. That way, you can pick your frameset based on those two measurements as opposed to top tube or seat tube lengths which 'may' change depending on where they are measured or what type of seatpost is used.
As you noticed, by measuring the way that you did (from the saddle), the reach changes with every adjustment. So to maintain your desired position, you will either need to find a way to extend the extensions 5cm or move the entire aerobar forward 5cm when you go from a UCI to non-UCI saddle position.
Brandon Marsh - Website | @BrandonMarshTX | RokaSports | 1stEndurance | ATC Bikeshop |
But, measuring stack and reach from the center of the bottom bracket standardizes the measurements regardless of what brand or type of bicycle you ride. The point from which stack and reach is measured is static. It is ALWAYS the bottom bracket. That way, you can pick your frameset based on those two measurements as opposed to top tube or seat tube lengths which 'may' change depending on where they are measured or what type of seatpost is used.
As you noticed, by measuring the way that you did (from the saddle), the reach changes with every adjustment. So to maintain your desired position, you will either need to find a way to extend the extensions 5cm or move the entire aerobar forward 5cm when you go from a UCI to non-UCI saddle position.
Brandon Marsh - Website | @BrandonMarshTX | RokaSports | 1stEndurance | ATC Bikeshop |