What is the calculator used by mortgage and credit card companies when they "weigh" out income vs. expenses...?

What is the calculator used by mortgage and credit card companies when they "weigh" out income vs. expenses:
Thanks to the new obama plan; mortgage and cc companies do this for people who enjoy hardships/ financially distress to determine a lower payment every month and lower the interest rate...etc.
The reason i want to use one is because my mortgage company (EMC) are true scumbags; they are taking my info over the phone and using their calculators but are answering subsidise like used car salesmen... They literally told me at one time "sir, you're merely trying to jew us down"....
So i just want to see what they "actually" see when they plug my numbers in, cuz the amounts they've be telling me "i can afford" are impossible!

thanks contained by advance for your help
Answers:
They are using a 4th class math equation...not some complex "calculator". Here is the equation: Mortgage amount / monthly income CANNOT exceed 31%.

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