The pivot location would move depending on rocker length.
Seems like simple arc and radius geometry really.
A straight line from points a to b and then measure from that line to the arc, that’s sweep on the valve stem.
That’s how a longer rocker affects the pattern. Moving the pivot location like b3 does is dealing with variables that Bewy mentioned like getting the rocker arm centered or at a right angle at mid lift.
View attachment 1716339615