I am a strong proponent of the 50-yard zero for the Eotech's sight height (7.2cm over bore). According to iSnipe, the trajectory looks like this on a cold day (a warm day would give you about a half-inch less drop at 300) using 55gr ammunition with a ballistic coefficient of 0.243:
Yards................Point of Impact
5 yards....................2.5" low (use the bottom of the ring to aim)
10 yards...................2.2" low (use the bottom of the ring to aim)
15 yards...................1.9" low
20 yards...................1.6" low
25 yards...................1.3" low
30 yards...................1.0" low
40 yards...................0.5" low
50 yards...................right on
75 yards...................1.0" high
100 yards..................1.7" high
125 yards..................2.1" high
150 yards..................2.1" high
175 yards..................1.8" high
200 yards..................1.1" high
225 yards..................0.1" low
250 yards..................1.7" low
275 yards..................3.8" low
300 yards..................6.5" low
350 yards.................13.7" low
400 yards.................23.7" low
450 yards.................37.0" low
500 yards.................54.2" low
Here's a 100-yard zero instead, under the same conditions as above:
Yards................Point of Impact
10 yards...................2.4" low
25 yards...................1.7" low
50 yards...................0.9" low
75 yards...................0.3" low
100 yards..................right on
150 yards..................0.4" low
200 yards..................2.3" low
250 yards..................6.0" low
300 yards.................11.6" low
400 yards.................30.4" low
500 yards.................62.7" low
You can see that the 50-yard zero puts the bullet at the line of sight a little sooner, while also giving you considerably less drop beyond 200 yards, at the very small cost of being ~2" above the line of sight between 100 and 150 yards. I think that's a good match to the capabilities of both the .223/5.56x45mm cartridge and the Eotech as a sighting system.
What's the EOTECH like shooting at 300 yard targets? Is the target partially or entirely covered by the dot?
The optical limit of a human eye with 20/20 vision is 1 MOA (1 arcminute). So in digital-camera terms, a 3-inch target at 300 yards will be about one "pixel" wide to the human eye, a 6-inch target will be about two "pixels" wide, and an 18x18-inch target will be six "pixels" wide by six high. The Eotech dot at low brightness will appear one "pixel" (1 arcminute) wide at all distances and under all magnifications, so assuming no magnification it would completely cover a 3" target, half cover a 6" target, and obscure about one sixth the width of an 18" target. Of course, someone with 20/40 vision will see the Eotech dot as being twice as large as someone with 20/20 vision sees it, so if your vision is worse than 20/20 then the dot will obscure more of the (blurry) target. Ditto if you have astigmatism or are looking through the edges of thick glasses, which is a problem with all optics.
In my experience, the greatest difficulty in shooting at 200+ yards with an Eotech and no magnifier is difficulty seeing the target clearly, rather than obscuration of the target by the Eotech's dot.