In fact you cannot do it accurately.
You can presume the dynamics of reaction force has the same ramp up time as the PSV.
To the second question, the time when PSV is fully opened depends on the quantity to be released- probably you'll never know it.

IMO opinion, you are not on a conservative approach and the question is why you try to avoid a DLF of 2 or a DLF established by a zero ramp-up?