Unfortunately, such a caption position (about 20% down from the top of the image justified to the right side) is not supported.
The caption can be positioned only via the captionPosition configuration option (OVERLAY, OVERLAY_IMAGE, BELOW_IMAGE, BOTTOM, BELOW_THUMBS, NONE).
Trying to change the caption position via CSS would be very difficult to do (if at all possible) and even if you were able to do so, you may run into unforeseen problems when the gallery is redrawn (for example if the user changes the browser window size) as Juicebox would not know of the caption's new position and would expect it to be in one of the official caption positions.
I hope you understand.