If this person is not responsible to pay a note she is not responsible to pay rent. Besides, your mortgage payment has nothing to do with how much rent will be paid. You need to research your market, find out what your place rents for and fix the rent. Maybe this person is not the one that should be your tenant. Again, if she can not pay the note, how will she pay the rent? Do not allow the tenant to tell you what the rent will be.