How much aliment should i expect to pay

My wife and I plan to seperate...
we dont have a mortgage, we live in a housing assosiation scottish secure tenancy at around 300 per month...
we have one child together and i have a child of my own which i pay maintainance for(130 per month)...she also has a child of her own which her ex pays for(130 per month)...
....I have agreed to take all of the debt on myself totaling around 200 per month in minimum payments
I have agreed she can keep the car (£2000)
and all the houshold possetions
she is 32 and i am 26
she does not earn as she has never worked in our marraige
I earn a salary of 30k per year
upon leaving i have agreed to pay all the bills until appropriate benifits kick in for her(around 2 months i expect?)....at which time i will start to make aliment payments... unfortunately I have no idea on what kind of amount would be appropriate
any one have a ballpark figure in mind...

I want to be fair

I was thinking around 500pounds including child maintainance...which would be 25% of my salary and after benifits...would leave her better off than me by around 100 per month...

any help would be much appreciated

really dont want this to be messy as all I care about are the kids having what they need.