It's good that you realise however that it is just a suggested starting point for further adjustment.
My system stages and images quite well but my tweeters are underneath the dash pointing directly towards each other. To be completely honest, the only way to find the best position is lots and lots of testing. A good way is to get your tweeters, stick a bit of blue tac on the back, hook them, grab a track which tests the imaging/staging and you know very well, and then just try lots and lots of positions to get the effect you're looking for. And every 15 minutes, take a break or else your ears lose their ability to accurately distinguigh subtle differences...