How do I account for pre-tax deductions from my income in a cash flow analysis?

It is important that you do not double count these expenses deducted from your income (like health insurance premiums or retirement) by including them in your budget AND deducting them from income.  You should only account for them once.  If you include them in your budget, they should not be deducted from your income in the support calculation. If you  account for them as a deduction to your income, then they should not include them in your budget. Either way works - but be careful you don't account for them twice.