Had to look that one up, Robin! It's not in the plans - it's in the manual.
It's on SK-54, which shows it to be centered vertically and 18 inches forward of the inboard edge of the leading edge of the horizontal stabilizer. At least that's how I interpret it.
Also, there's another SK-54, that one for a detail of the forward canopy, just to add to the confusion.
Well done, on the quick response. Also, thanks to Paul and others for staying active in the "good 'ol RV-3" forum, even though you're flying and there aren't too many people building 3s..