well, you could try with the DF at around 10 or 10:30pm, the same amount that she takes at the night feed. feed in the dark, without waking her. if it works- than great. but if it doesn't, don't worry and just don't continue with it. it may take some time for your LO to get used to eat [I mean at the beginning she may not want to take it at all or would still wake at night] so if you decide to do DF, give it at least a week before you decide if it's working or not.
hth
Monika