Mathematically, =POISSON(x,m,TRUE) is equivalent to each of the following
expressions
=CHIDIST(2*m,2*(x+1))
=1-GAMMADIST(m,x+1,1,TRUE)
So you can use CHIINV and GAMMAINV to obtain Poisson confidence limits.

Numerically, this will fail if m needs to be too large, because of
weaknesses in Excel's implementation of CHIDIST, GAMMADIST, and their
inverses.

A world-class (for both accuracy and working range) library of VBA
probability functions is available in
http://members.aol.com/iandjmsmith/examples.xls
which also explicitly includes confidence limit functions for discrete
distributions.

Jerry

"SteveG" wrote:

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?