Generally electronic locks (magnetic levers) should be changed at 50% where as deadbolt (motor) locks should be changed around 75%. This is because of the load the lock/unlock actions put on the battery cause a sudden voltage drop and if the battery capability is diminished, those sudden voltage drops can translate into the voltage falling below the minimum operating voltage of the lock and render is dead suddenly.
So while it may continue to function below these thresholds it can be very unpredictable and have unintended consequences. E.g. if the temperature suddenly drops outside, this can cause the battery voltage to further drop and shutdown the lock.
See this first post image for thresholds we’ve seen for different types of devices when we recommend changing batteries.
See this post for an examples of voltage curves and how suddenly voltages can drop. Remember when the current draw “load” increase, these voltages can drop much faster than the curve shows. Hence for deadbolt locks which have a big draw even at 70% battery with rapidly falling temperature it can send the lock into shutdown mode very quickly.