I am trying to estimate the number of spare parts required to give say
95% protection level.

At the moment I have used the Poisson function to calculate the
protection level with various number of spares. I then use MATCH to
find the first value that exceeds the required protection level.

Is there a way of reversing the POISSON calculation so that I input
protection level plus Mean demand (cumulative = True) and output the
number of spare parts required?