Thanks very much Mark, this is clear enough, so far !

To progress a little bit further I just need to find 1) what is the HUD center point position (x,y,z) relative to the standard flight simulator center point and 2) the HUD radius. Those values are all given in feet.

the eypoint format is y,x,z (feet)
the collimation format is x,y',z,radius (feet)

in both formats x and z are the same, the y value is not (noted y'). Is it always like this, whatever the aircraft ?

Let's see 2 examples :

CS F/A-18 eyepoint -17.00,0.000,3.358 collimation 0.0,14.01,3.3580,0.307
F/A-18 + mod eyepoint -17.00,0.000,3.380 collimation 0.0,14.40,3.3800,0.310
for both these a/c the standard FS center point is 29 feet forward of the datum_reference_point

Last question : the CS Weapon VC hud3d width, height, hor. size, vert. size are all given in pixels. How can I convert feet in pixels depending on the monitor resolution ? mine is 1920*1080

PS just found how to convert feet in pixels with the ppi = pixel per inch formula

http://en.wikipedia.org/wiki/Pixel_density