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ST angle by changing the offset of the seatpost and/or using the adjustment scale built into saddle rails.
yes there is adjustability but not as much as stack and reach with more variables (spacers, stem length, bar shape/reach); what @Maj said
no1 should base their fit on ST angle, its just that stack and reach are seen as a gospel but they are just one of the factors and not even that important
fuck stack and reach, the worse way to figure out IMO, its all 'reach' based and biomechanics change all the time, your core can get stronger/weaker, supporting muscles the same. IMO ST angle is more critical. The rest is 'ride feel'.