Under the Home Affordable Modification program, the target maximum amount for your mortgage payment (or mortgage debt-to-income) should be 31% of your gross (pre-tax) monthly income. This Payment Reduction Estimator will determine what your current mortgage debt-to-income is and how much your monthly payment may be reduced if you qualify for a modification.
Do not include any payments on your second mortgage. You may have taxes and interest in escrow added to your monthly payment already, so be careful to count taxes and escrow only once.