Normally the accumulator pressure should be about 90% of the system pressure. This way the gas volume will occupy much of the accumulator volume, but it is the gas that stores the energy. The problem is that this can leave little oil in the accumulator. What must be done is to calculate how much oil will be used from the accumulator and multiply that by 13.8 to get the accumulator size.
I have a program that does all the calculations and simulations for me.