It sounds as though you are trying to study the topic of budgeting and
forecasting, rather than wanting to ask a specific Excel question.

I think you need to work out what you want to accomplish (i.e. talk to the
managers on how they do their budgeting) and then possibly look at using
Excel to assist with the task. Don't start with a solution and go looking
for a problem.

In summary ... talk to your managers, they already know how to budget and
forecast.