How much money should you have left over a month?

My husband and I are wanting to buy our first house instead of rent! After the new mortgage,insurance,car payments,groceries, utilities, daycare every expence we could think of we will have anywhere from $450-$1000 left over a month. I am just wanting to know if that is an okay amount of money left over after all expenses are paid I do not want to be "hanging on" and barely scrapping by. We do have some money in savings for emergencies but I try not to count that for We only want to use that for fall back!
Added (1). My main question is will $450-$1000 be a safe fall back of left over money for a month?

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