Compared to the countable liquid spiritual power at the ninth level of Foundation Establishment mentioned in the Pure Yang Scroll, the Divine Wood Sect uses approximations.
Basically, when you feel that your Dantian Qi Sea has been filled with spiritual power and can no longer condense and elevate, you can start breaking through.
Whether the breakthrough will be successful depends entirely on fate.
If you overcome the bottleneck, your Dantian will expand, allowing you to condense and store more spiritual power. If not, one must heal the damage to the Dantian and try breaking through again after nourishment and recovery.
In contrast, the experts of the Immortal World have standardized the pattern and the amount of spiritual power required for each breakthrough.
These are outcomes of endless experiments, and they represent the easiest way to break through and the way that minimally damages the body and divine sense, even if the breakthrough fails.