  The correlation length $\xi$, a key quantity in glassy dynamics, can now be
precisely measured for spin glasses both in experiments and in simulations.
However, known analysis methods lead to discrepancies either for large external
fields or close to the glass temperature. We solve this problem by introducing
a scaling law that takes into account both the magnetic field and the
time-dependent spin-glass correlation length. The scaling law is successfully
tested against experimental measurements in a CuMn single crystal and against
large-scale simulations on the Janus II dedicated computer.
