HYDAC recommends the following:

For energy storage: Po = 0.9 x P1, (P1 = minimum working pressure)
For shock absorption: Po = (0.6 to 0.9) x Pm, (Pm = median working pressure at free flow)
For pulsation dampening: Po = (0.6 to 0.8) x Pm, (Pm = median working pressure)