I've found it much more effective to aim the centre of the crosshair at the middle of the wooden structure he's on; the mid-left crosshair should be on the left-hand guy.
As with most siege weaponry, the goal is to hit the structure, not the individual.
After trying 10 times to hit him, I just aimed for the entire thing and got it first go.
I'd also reccomend making a quicksave just before it (after Foltest talks to the arbalist) and hit reload whenever Geralt doesn't order "One and a half degrees".
Good luck, I found this to be a major hassle too!
gaming.stackexchange.com/users/9708/YMSDC
According to the Game Guide that came with my copy of the game, 1.5 degrees is the right angle.
I've got my brother testing what that does currently, and will update when he succeeds (or fails).
Edit: It appears that if you get the correct angle and kill the "red-plumed man" you will not have to fight Count Etchverry after breaking through the barricade.
(so it reduces the number of enemies you have to fight a little later on).
gaming.stackexchange.com/users/336/Raven Dreamer
For those having trouble with this, the wall has 2 openings\windows and 3 pillars.
Aim the middle of your spy glass at the mid-bottom of the furthest right pillar.
You will succeed when your character says 1.5 degrees.
gaming.stackexchange.com/users/9768/lavis
The right angle is 1.5 degrees.
It means that you need to have the target between centre and upper line.
If you play on a, PC you need to use arrow keys in order to aim correctly (saying that just in case CD Projekt will release the game on other platforms).
If you use the mouse, Geralt will constantly say "3 degrees".
Good shooting. :)
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I found that aiming lower or dead centre of the platform didn't work for me.
Instead I had to aim at the guy without the flume on the right.
This was the only way I could get the correct 1.5 degree call.
Oddly in my game it seem that the horizontal angle mattered more then the vertical.
gaming.stackexchange.com/users/257950/Isaac Allwood
In the tutorial, the king gives Geralt the spyglass to aim for the "man with the red plume", while he adjusts the ballista. I felt like I had the cross-hairs right on the man, but when the bolt was fired, it was a bit high.  Is it possible to actually hit the target? If so, do you need to aim somewhere other than directly at him (perhaps a bit low)?
In The Witcher 2, is it possible to hit the man with the red plume?
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gaming.stackexchange.com/questions/22540
gaming.stackexchange.com/users/816/sjohnston
1.5 degrees is the right angle. The wall has 2 openings and 3 pillars. Aim the middle at the mid-bottom of the right pillar.  The right angle in order to hit the man with the red plum is 1.5 degrees.
The right angle in order to hit the the man with the red plum is 1.5 degrees.