Excellent decision on telling your daughter she is both! I've done some research on this subject and apparently many people tell their child that they are half Japanese and half American (or British..etc) This leaves the child feeling that they are not either. I also feel that being a dad in Japan has a tendancy to take you away from your child therefore resulting in the child favoring Japanese culture more and rejecting the other at first. It is really important to be an active part of the childs life. ie: reading at bedtime, talking one on one, bathing...etc.
Sometimes I also wonder if maybe if just us parents are the ones really taking so hard...
Hope I don't sound like I am telling you how to raise your child...just some interesting info.

日本語では、両親の国籍の違う子どもを「ハーフ」と言いますが、イランでは「二倍の血統」という言い方をするそうです。
二つの国の優れたところを受け継ぐ、素敵なお嬢さん達だと思います。