Oh! that explain things. I was trying to project the position of an object from an undrotated layer to a rotated one, and as it was moving clockwise, I thought "ok just have to counter-rotate when changing layer so currentAngle-LayerAngle should work"
And to my surprise it was currentAngle+LayerAngle
Now that I know that it's the layout that rotate, it makes more sens.
Anyway, if you wonder how is percieved this "layerAngle" feature, I thought that the main goal of it was to rotate grouped sprites (grouped under the same layer). So yeah if I want to rotate grouped sprites 90 clockwise, I would think that the layer should rotate 90 clockwise. (And in graphic apps like photoshop it's how it is done)
Also, even if it's a bit away from the point, following this logic, it would be intersting to be able to set the center of rotation, and also to be able to move the layer wherever we want.