Add up ALL expenses you have during the month, use the past year's statements to look through your history and see if your expenses have gradually gone up monthly as well too, so you can plan on them to keep raising. After everything is added up, how much do you have leftover? Figure out how much of that needs to go into savings, and how much you can use towards a monthly mortgage payment. This should give you a pretty good idea.
I also agree that if you have two incomes, plan using just one. In today's market, it's highly probable that one job could be lost and you want to be prepared!