The balance, and the one minute it takes
The balance is the second half of the figure, handed over when she arrives, before the session starts and never in pieces along the way.
Whatever did not go ahead is paid on arrival. That is the whole of the rule, and it is deliberately dull. She knocks, you open, the sum is handed over, and the evening proceeds without money being mentioned again.
Why at the start rather than the end
Because a conversation about money afterwards is a conversation nobody enjoys and everybody remembers. Settling it in the first minute takes it off the table for good. It also means there is no counting, no dividing, nothing paid by the half hour and nothing added on at the end. One sum, once, at the beginning.
Cash, and other currencies
Cash is what this half is normally paid in. If you are carrying another major currency and would rather use it, say so in the thread while the hour is still being agreed, and the exact amount in that currency is confirmed to you in writing before she leaves. That way there is no arithmetic done on a doorstep and no disagreement about a rate. Every figure written on this site is in euro, and the conversion is a courtesy rather than a second price list.
Nothing is added afterwards
Her journey across the city is not charged to you. Nothing is added for a weekend, for a public holiday, or for a late hour. A session that ends earlier than the booked time is still paid at the booked figure, since the time was held for you and cannot be offered to anybody else at that point. The full ladder of figures sits in each profile on the roster.