I figured out why they are using net income and how they came up with the number. The guidelines read:
Monthly net income can be used for preliminary screening and qualification. If used, the servicer will need to multiply net income by 1.25 to get to an estimate of Monthly Gross Income.
31% of my net income X 1.25 is exectly the payment amount they gave me.
I expect the final modification (31% of my actual gross) to be a couple hundred dollars a month more than the trial period payment. I wonder if I'll have to make a payment for the difference?