This is where my Knowledge Stats < Knowledge Excel! (I inherited a
spreadsheet using Poisson.)

In the poisson calculation I have Probability = POISSON(x,m,TRUE)
where x is the number of spares and m is the 'demand'.

Ideally I would reverse this calculation inputing the required
probability and obtain x (the spares level required).

What you show is Probability =CHIDIST(X,degrees_freedom)=
CHIDIST(2*m,2*(x+1) or Probability =1-
GAMMADIST(X,alpha,beta,cumulative)=1-GAMMADIST(m,x+1,1,TRUE).

When I turn to CHIINV(probability,degrees_freedom) or
GAMMAINV(probability,alpha,beta) I still see the degrees of freedom
or Alpha parameters as inputs when they contain the x number of
spares.

This leaves me confused ...

.... But fortunately in the examples.xls sheet I see the
crit_poisson(B70,D70) calculation which fortunately seems to match my
original calculations!

So I will probably use this!

Thanks
Steve