Do not make beginning depend on finding every number.
Start with the income you receive, the biggest regular costs and the debts or savings that matter to the question. Use rounded estimates for the rest and label them.
Let the first scenario ask better questions
A rough result might show that rent matters far more than three small subscriptions, or that one missing annual bill could change the answer. That tells you what to find next.
What to try next: Give yourself 20 minutes. Build the simplest honest version you can, then stop and notice what you learned.