expodisc and on-camera flashgun

I have just bought a expodisc and I must say it works beautifully.
But, how do you use it when you have the flashgun mounted on the camera?
Normally you take the camera with expodisc mounted to the subject and then point the camera at the point where you will have the camera when taking the actual picture.
In natural light and with studio lighting this is easy, but how do you set the whitebalance with an expodisc with a flashgun mounted on the camera?

Use a mirror
Matt