Stack and reach are only the start
Why the same frame size can put your bars in a different place.
Compare dimensions, not size labels
A 54, 56, or medium label is not a cockpit coordinate. Frame stack is a vertical measurement and frame reach is horizontal, both from the bottom bracket to the top-center of the head tube. Use the geometry chart for the exact model year and size. Do not substitute effective or grip stack and reach.
Move from the frame to the clamp
Total steerer rise is measured along the steerer to the stem-axis center. Include the headset cover, spacers below the stem, and the stem clamp-center offset. For a symmetric clamp, the last term is half the clamp height. Do not add spacers above the stem.
A worked example
With 560 mm stack, 385 mm reach, a 73° head angle, 40 mm total steerer rise, and a 100 mm stem at −6°, the reference clamp is approximately 471.5 mm forward and 617.3 mm above the bottom bracket. These are synthetic inputs, not a named bicycle or recommended setup.
What an identical clamp position does not prove
The same clamp coordinates do not establish the same hand position if handlebar reach, sweep, rotation, or hood placement changes. Saddle position, crank length, rider mobility, and handling are outside this calculation. A fitter or mechanic must verify the complete setup.