When you are on a budget, be very conscious of it and know how much you're spending and how much you can afford and how much is too much. The point is, have a firm budget. But that doesn't answer your question, particularly when you're planning on how much you need. Those two things will be the biggest single chunks of your cost. Big cities always charge a hefty premium especially in the tourist season.
Food is a very variable expense.
Then there's the touristy expenses, and these are by far the most variable. I'm talking of things like tours, museum admission, bike rentals, paragliding excursions. You have to decide one way or another what you want to see, what is worth it and what is not. Then again, in youth hostels you can often find great bargains for these sorts of things. Finally there are the emergency expenses.