The FL of an eyepiece is not measured from anyplace in particular, unless it is
a single element EP. The EFL is based on the magnification with a particular
objective, not a measurment from somewhere in it. There are formulas for the
EFL of an eyepiece, depending on the design and glass used. The formula for a
2-lens EP is pretty straightforward (using the FL of each single lens and the
distance between them), but adding more elements complicates this. Generally,
the FL printed on an EP is an approximation.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)