You can't precharge it in the presence of system water pressure. The objective is to have the air bladder full of air, ie occupying the entire steel tank, at system cold pressure. If there is any pressure on the water side while precharging, the calculated or targeted precharge will result in the air bladder being less than the full volume of the tank, thus reducing its capacity.
If the pre charge is too high or too low you'll get PRV bleeding when the system is hot and auto fill filling when it's cold. But you can't tell which problem you have ... Too high or too low because both result in reduced air volume for expansion.
If the tank is connected and charged too high you have to remove air and add water, or if too low you have to add air and remove water in an amount corresponding to the volume required for the bladder to completely fill the tank. There is no practical procedure to do either of those simultaneous adjustments, and unless the tank is made of glass you can't even know which one is necessary.
The only way to do this is with the water side of the tank open to the atmosphere.
This Answer and this article are good further reading.
