There are many factors that affect the requirements, some of them are:
We always charge at least 20%. It's because we don't want to be the reason people get stranded if they have to use the car urgently and you have an all-electric car.
The savings must be more than 5% or the smart charging will not start. This is because it is better for battery health to have an ongoing charge instead of pausing and starting "unnecessarily".
If there is too little time left on the charge before the car is fully charged, the smart charging will not start. For the same reason as above.