Wow, You rock. A great explanation, thanks. I didn't know any of that. Maybe those squares could help a bit. I didn't know about the scatter inside the boxes.
No worries!
Juggling with in-game distances feels always a bit wonky, since usually there's no giant ruler laying around to put these dimensions into perspective. But this may give a slightly better impression about the actual size of these boxes compared to the ability's indicator:
As it turns out, the width of the indicator is already correct (8 m) and the 21 little chevrons inside can be used to quite conveniently gauge the length of the scatter box(es). The first rocket will impact somewhere between the 4th and 6th chevron, the second between the 7th and 9th, and so on and so forth. Just keep in mind that the area between the 1st and 3rd will be outside the abilities range while aiming...
Damn, I always had the feeling that placing the barrage right on top of your main target (for example clicking directly on an enemy MG leads to less damage and wipes. Now I know why, thank you!
Apart from that, I remember that some pro players had the rule of thumb that the first rocket will approximately impact on the first arrow of the overlay. Not sure if that is true, but maybe that helps some people as well as it is rather easy to follow if you want to hit two distant targets at once (which the Stuka is perfect for)
Heh, it was after reading your comment about these little arrows that I had to go back and check how they align... The overlay above isn't perfect but it seems each group of three - expect the first set - line up pretty nicely with the scatter boxes for each rocket. So as a rule of thumb, I'd recommend using #5, 8, 11, etc. as visual guide where the rockets will most likely impact.
Regarding the optimal spot to place the barrage... I've run some calculations the other day and, well, things are a bit complicated. For now it seems against infantry, it's indeed best to place them right in the middle of one of those scatter boxes to maximize the wiping potential. I'll probably post a more comprehensive analysis some other time if anyone is interested.