Most depth of field calculators use a simplified thin-lens model, so the ideal reference is the lens’s front nodal/principal point. In practice, you usually can’t locate that exactly, so measuring from the front of the lens is a common approximation and gives a conservative DoF estimate.

Be aware that lens manufacturers usually quote focus distance from the sensor/film plane, not from the front of the lens. That’s why those numbers won’t match a calculator that assumes a thin lens.

For normal shooting distances, this difference is usually small and not worth worrying about. At macro or very close distances, the mismatch becomes more noticeable, and calculator results are only approximate anyway.

So in practice:

• use the front of the lens if the calculator expects a thin-lens distance,
• use the sensor-plane mark on the camera body when comparing to lens specifications,
• treat DoF calculators as guidance rather than precise scientific tools.

If your calculated and real image sizes differ a lot, close-focus behavior and real-lens design are likely part of the reason.