I just saw a really great tutorial about setting the wb in Nuke at the Foundry site. We do this using the grade node.

1. Add grade node to the image you wish to correct the white balance in. In the blackpoint enable the eyedropper and Shift + Alt + Ctrl drag an area you consider as the darks in the skintones. It is important that we use all 3 modifiers to sample the raw data of the image not the data with the adjustment added to it. ( this will cause a “loop” and the image will flicker in the viewport).
2. Repeat the process with the whitepoint and sample the highlight area of the skin you want to match.
3. Now we start sampling the clip we want to match to. In the lift sample the corresponding dark tones in the image we want to match the wb with. Note we don´t need to hold the alt modifier since we´re not grading the image we´re sampling.
4. Do the same with the gain. Sample the highlight area. Done !
5. We can do some additional color adjustments to get even closer.