Use dates rather than a single average
When income arrives unevenly, a monthly average can hide the weeks in which essential bills are due. Draw a calendar showing expected receipts and the costs they need to cover. Separate confirmed income from an estimate. If you use earlier months to describe a pattern, explain which period you reviewed and whether the current situation differs.
Make the low point visible
Identify the part of the calendar with the least room after essentials. Add the evidence behind that observation: a pay statement, a recurring bill, or a dated record of variable receipts. The exercise is not an IRS financial determination. It is a way to describe the timing problem clearly before someone considers what further records or calculations are required.
Plan the question, not the approval
Ask how a proposed payment arrangement would fit the dates on your calendar and what responsibilities would continue. Keep current-year tax needs on another line so they are not absorbed into an older-balance payment. If the pattern changes, retain the earlier calendar and date the new version. A practical discussion should explain the assumptions being used, the documents still needed, and what you should do if the expected income does not arrive.