The single measurement that makes this piece usable is the setback - how far the hutch sits behind the front edge of the buffet below it.
Get it wrong and the furniture fights you. Too little setback and you cannot put a hot dish down without your forearm coming up against glass. Too much and the buffet top turns into a shelf you have to lean over, and the whole cabinet looks top-heavy and pushed back.
Our artisans set it by arm, not by rule. Someone stands at the buffet at serving height and goes through the motion - lift, place, slide back, lift again - while the hutch is held at different depths behind them. The setback is fixed where that motion is comfortable for an average reach, and then the moulding that runs along the front of the buffet top is designed to that same line so the joint between the two sections looks deliberate rather than left over.
The carving on the crest is set out afterwards, because the crest has to look centred over the hutch as seen from where you stand at the table - not from directly in front.