Poisson distribution

New to macro in excel. Need to implement Poisson distribution in a macro. The attached sheet has details. Need the stock level for 80% 90% 95%. So basically a number in k5 that will generate 80% Poisson Probability. This can be done manually ofcourse. Need to automate this process for the values in columns L M N.Obviously ignore 0 cells in those columns.


For the first case stock level 2 is close to 84% instead of 80% which will work. But there wil be no stock level for 90% and a stock level of 3 fore 95%.

In short is there a way to fill out Q R S through a macro?