There is no way to "measure it", just work around until you find it, or import the player model into your modelling program and adjust stuff.
I did import the player model into milkshape and moved the gun to the center of his head but any movement in milkshape i make does not translate into blockland at all. An example would be that if I had to move the gun 5.59(x), 1.9(y), and 0(z) if I wrote that down for the eye offset (swapping the y and z coords of course) it would not line up at all. I looked at HellsHero HK417 and his offset has like 2-3 decimal places. I'm wondering how he came up with that.