Embedding Uncertainty into Excel

Hi guys

I am attempting to plan out my funds for the future as I just got a new job. I have successfully created a sheet, but now I want to mix things up. I want to add the probability of recessions into the mix, so that my expected market returns can, and will, change. How would I go about this if I want the probability of a recession (minor or major with market drops say 10% and 40% respectively(the numbers dont really matter)) to be embedded into the code.