The recommended pressure for the nitrogen in the T137 accumulators is 550psi.
A true nitrogen pressure check can only be competed with no hydraulic pressure on the other side of the accumulator.
To set the pressure an accumulator pressure test kit is required.
A kit to complete this is sold by Charlatte of America as part number H00706
It should also be noted that the accumulators sold by Charlatte are not set unless the customer asks for this to be completed.