Here's the problem. Lenders have their own means of determining your income, depending on how you receive it and how it's changed over the past 2 years. When calculating your debt, they often rely on what the credit report states the minimum payments are for various accounts. This information isn't always accurate. If no payment is indicated where a balance is owing, they will guess, which may not be right.
For you to do it as good as any online calculator take your total minimum payments for any credit cards, auto loans or other loans you have and divide it by your average gross monthly income. For example if you owe $500 per month on a car payment and $200 on various credit cards, you have $700 in debt. If you make $5000 per month, divide 700 by 5000 and you get 0.12 or 12%. Another way to do this is assume your lender allows a 43% combined debt to income. 43% of $5000 per month is, $2150. If you subtract the $700 in debt, you can have a total mortgage payment of $1450 including Principle, Interest, Taxes, Insurance and Home Owners Dues.
Now when you take your calculations to your lender and they are different, remember I told you the bank will calculate your income and debts differently. If you can show your minimum debt payments are less through a statement from a credit card or bank, you can get your DTI adjusted.
I hope this helps.