We do this with our boys already (they are 10 and 9) - we gradually increase the money each year but also increase what they are responsible for buying. Personally I would track what you are actually spending on her already and thats the amount she should receive. For example what about Haircuts, presents for friends, trips to the cinema? etc. Hoe much realistically do you spend on her clothes/shoes, will you expect her to buy school books etc. Once you have decided on an amount and what she is responsible for, I would get her to track what she is spending on what ceratinly for the firts few months, then you can see whether what you give her is reasonable or not. You need to remember that there are some things that she wont necessarily need to buy in the firts few months that she will need to keep money put by for e.g. new school stuff for the new term.
It is a great way to teach financial responsibility and to show them the value of money. Good luck with it. There is a good book on doing this but the name escapes me. I'll look for it later and let you know.