Measure from a known state of wind
Approximate reserve is more useful when you know the watch began fully wound rather than after an unknown amount of wrist wear.
Compare manual winding with wrist winding
If a watch runs a normal duration after manual winding but performs poorly during wear, the automatic winding system deserves attention.
Friction can shorten reserve
A dirty or worn movement may consume energy inefficiently even when the mainspring stores a normal amount.
Movement specifications vary
Do not compare every automatic watch to the same expected reserve. Caliber information helps set the right baseline.