Are you accounting for all the spending you have to do over a year? So not just your monthly bills but car maintenance, home maintenance, replacing technology, haircuts and personal care, birthday presents, Xmas presents, etc. As that will easily add a couple of hundred on to your monthly expenditure, if you're saving/accounting for those. And if you're not, do you have ample savings to dip into for those if needed?
I account for these expenses every month, and they add a fair whack onto monthly expenditure but means that I don't have to find large sums of money out of one month's income so it smooths things out. For example by Dec 1 I will have £600 in my Xmas budget. My car maintenance fund gets £50 a month. By the time it's DD1's birthday in Feb I'll have a pot with £175 in it for that. Same for DD2 in June. They tick away every month gathering whatever amount they need to stay on track.
I think often when people are talking about their monthly outgoings, they totally fail to account for all the things their money has to do that isn't necessarily a monthly expense, but has to be paid for from somewhere. And that's when you get into trouble, because you end up scrabbling to cover big expenses out of your normal monthly income, which doesn't work if your income is fairly tight.
In terms of figures, excluding mortgage we account for £2800 a month, but that covers almost everything in our budget, including saving for future goals.