Your income can fluctuate for a variety of reasons, apart from working in a commission based setup, like investment income, bonuses etc.
In such cases, you should plan for your average income over the past few months.The InvestmentYogi budgetting spreadsheet provides you with an average for each income source and expense head, and this can be a good basis for planning your budget.Over time, any excess income you earn will carry over to future months, allowing you to maintain a standard expense level, while also allowing you to plan for your financial goals, investments etc.
Happy Saving! :)