However, in your spreadsheet you added the dispersions quadratically.
True. That was easy way to sum them up. Then I just used normal distribution to stress center values. I'm not saying it's just equivalent with reality but at least it's enough for comparison. If you have something better in mind I'm still interested.
Although many dispersions are indeed independend, at least two of them are a bit correlated. If a bullet leaves the muzzle at a higher than average speed, it will hit the target higher and will be deflected less by wind. It is a rather minor effect, barely noticable, but never the less present in my simulations (in the WhatDoIShootToday spreadsheet): bullets that are deflected less hit the target on average higher and vice versa.
True. BC variation goes the same way. I have excluded those as small errors. Regarding muzzle velocity variation there is at least as important effect that's very hard to predict. That's muzzle velocity variation's dependence on zero range. This effect is introduced by barrel vertical vibration and barrel time. This is the reason why best benchrest rifles can shoot 1000 yard groups with nearly no vertical. More information about this here:
http://www.varmintal.com/aeste.htm and ladder test which makes use of this effect:
http://www.6mmbr.com/laddertest.htmlIt would help if you could give me the page number, then I do not have to think to hard... 
It's page of 184 in the second edition of his book.
I think that's chapter 11 in the first edition. Here he speaks about the same thing:
"For this example, we’ll consider a 30-06 rifle shooting 185 grain bullets at an average muzzle velocity of 2850 fps. The rifle will be modeled as being capable of 1” groups at 100 yards on average. For this current analysis, we’ll consider the impact zone at 500 yards. According to the principles of bullet dispersion given in Chapter 11, we can extrapolate the group size from 100 yards to 500 yards based on the bullets time of flight. The bullets time of flight at 100 yards is 0.1084 seconds, and at 500 yards, it’s 0.6154 seconds. Since the rifle is capable of grouping into 1 inch at 100 yards, we can expect the dispersion to produce a 500 yard group of: 1 inch times 0.6154/0.1084 = 5.7 inches minimum."
http://www.longrangehunting.com/articles/applied-ballistics-long-range-shooting-bryan-litz-book-1.php