Everybody workd differently here and you will get a lot of comments about the actual amount. However I would do the maths a little differently.
I would work out the TOTAL household income, and then the TOTAL exenditure on mortgage, bills, etc etc. Including a decent estimate for food shopping, kids clothes etc. BUT excluding 'personal expenses' that you each pay for separately (like your and her clothes, tiletries, pressents, mobile phones whatever)
Then you will see how much is left after household expenses are paid, and you can decide how to split that. The fairest way is 50:50 so you each end up with the same disposable income.
But you do need to take into account 'big purchases' like holidays etc. and decide who pays for these (eg if you pay for then or does she contribute')
Go to the Money Saving Experts webpage and fill in the budget sheet, this really helps you remember everything.
In our household we do things slightly differently. I am teh main breadwinner, so I put £x into a joint account which pays all bills inc shopping etc. All the rest of my wages are MINE! which I use for things like my clothes, my pension, my mobile phone, my gym etc. DH earns a lots less than me, he pays for childcare, mobile phone, sky TV (his toy), pension etc. It ends out we both have (give or take £100) about the same amount of disposable income left. Big purchases are paid for jointly, or by him if he gets more work.
BTW you are not the dh of someone on here a week or so ago, worried about her dh not paying the Barclaycard bill over £1200 are you??