I would generally consider the size of the promotional increase given. Factor in the relative performance of her peers and how this would impact internal equity. I usually communicate to the person at the time of the promotion conversation that their raise is included in the amount (if you can't or don't need to give her more money).
Anything you say to her at this point is going to be perceived as a negative because she is already anticipating more money, so you're doing damage control now. I would explain that, since she just got a sizeable increase, you already factored this in. I would make sure this is the historical practice as well - you don't want her coming back with a precedent that undercuts your position.
Her annual raise was based on her past performance, but so was her promotion. She earned the promotion based (hopefully) on strong performance and by demonstrating the necessary knowledge, skills, and abilities to do the new job. We normally combine merit and promos if they're in close proximity time-wise, and let her know that her increase was substantial based on the norm.
Next time, you could convey this with the promo communication (x% is your promo, y% will be your merit). Make it easy.