Is it Possible to use Bayes' theorem

This may not be the right approach. With the help of other experts I have built an Excel sheet that provides the likelihood of a project finishing on a given particular day. My inputs are:

best/availability/productivity
worst/availability/productivity
best guess/availability/productivity


Once they are all divided and summarized a Cart. Product of 3 sets is used with Standard Dev and Var to determine the probability of the project ending on the dates estimated. This is working well. What I want to do is add a list of questions on the sheet where the values required will either be y/n or numbers (questions will be weighted according to importance) then see if Bayes can be applied to the sheet to improve the overall accuracy of the probability of completion.