I would probably start with 3/4 EBM and 1/4 formula and then gradually increase the amount of formula and decrease the EBM. Another thing is, are you the one trying to give her the FF? if so she is probably getting frustrated as she *knows* where her milk is and you are trying to give her something that smells and tastes completely different so it might help to let someone else try to give her the FF. If you got enough EBM stored, would you worry about using the formula at all?

ETA - tali, I think that's a myth about the nan formula, in my experience with it Nan has never made any of my kids *fat*