Eight oval chair backs are a test that a single chair never faces. One oval only has to look right. Eight have to look identical, and they are seen together, in a row, from the end of the table - the one viewing position that shows any variation between them at a glance.
An oval drawn freehand eight times will be eight slightly different ovals. So we cut one master template first, in board, and every back frame in the set is marked from that single template rather than from the chair made before it. Marking from the previous chair is how error accumulates: each one is close to the last, and the eighth is visibly not the first.
The template is cut from the client's confirmed seat and back dimensions, checked, and then kept for the life of the order. Every one of the eight frames traces the same line, so the ovals are the same oval.
We keep the template afterwards too. If a chair is ever damaged, or you want two more added to the set in five years, the replacement is marked from the original board rather than measured off an existing chair - which is the difference between a new chair that matches and one that nearly does.