Debt to income ratio is great because it determines what a healthy overall monthly payment should be.
If you make $4,000 per month ($48,000 per year) and you are buying a 4-family with potential rents of $1,000 from each unit, you have $3,000 in income and you will be occupying one unit.
You then multiple $3,000 * 75% = $2,250.
Add $2,250 to your income of $4,000 = $6,250.
Multiply $6,250 * .55 (max DTI for FHA) = $3,437.50
That is your total debt payments you can have on a monthly basis. You have to subtract minimum monthly credit card debt payments, car leases and loans, student loan payments, etc.
If you have $0 debt, you can qualify for a total mortgage payment of $3,437.50. This includes principal, interest, taxes, homeowners insurance and mortgage insurance!
Be careful using a conventional loan because you may not be able to use the rental income like the example above if you do not have a current housing expense (live rent free).