Don't go below 500 PSI on a phantom as a golden rule, beyond that is kinda of exploring uncharted territory. It should be fine on stock springs using 600-800 psi range with HPA. Below 600 you need to respring. With HPA your goal is consistency.
With 12 grams you want to squeeze as many shots as possible before drop off. Efficiency is stock class is very important. Even with 3.5-4 OZ tank you get 300-400 shots. That's more than all the paint I used to bring.