Also, it is much easier to not start spending money, than it is to give up expensive habits that you've acquired.

This goes double, once you're a family. Negotiating between partners and children about which things to cut back on is unpleasant.

My advice (which I've followed imperfectly) is that if you have the income to make this reasonable, leave a cushion where you can handle setbacks without having to make the hard choices.