OK I have a 20" flat top with a detach carry handle. I sighted so at 50m (or close to it) it is zeroed on the 6/3 setting with 55 gr. ammo.
Then took it over to 200 m. on the same 6/3 setting and got 3 consecutive hits.
50m zero on the 6/3 setting sound right?
Well, it's right if you say it is. There are lots of different ways to zero an AR. It is your rifle so you can do it the way you want.
The 50/200 yard/meter zero is very popular, providing a pretty flat trajectory out to a little beyond 200 yards or meters, with a rise or fall above or below point blank point of aim from about 10 yards to 225 yards of 2" or less. But that flat trajectory is a function of the ballistics of the 5.56x45 cartridge and has nothing to so with what setting on your rear sight elevation drum you happened to use when zeroing your rifle.
In other words, you could have zeroed your rifle at 50 meters using any setting on the rear elevation drum, and if you did not change that setting, you would have been on or very close at 200 meters.
The problem with your method, if there is one, is it isn't really utilizing the ballistic drop compensation built into the rear elevation drum of the A2 rear sight. That sight is basically designed to allow the rear sight to be elevated to predetermined settings to compensate for the bullet drop at ranges out to 600 meters. The markings on the drum correspond to target range in meters. The 6/3 setting refers to 300 meters with the peep set to its lowest elevation, and 600 meters if you rotate the drum one full turn higher. The "4" and "5" settings refer to 400 and 500 meters respectively.
The Army uses this sight to allow the rifle or carbine to be zeroed at 25 meters to provide a 25/300 "battle sight zero". So if you zero at 6/3 (with the elevation drum in the low "3" position) at 25 meters, and then shoot at 300 meters with the same setting, you should hit within a couple of inches of your point blank aim. You can now use the rear elevation drum to dial the rear sight peep up to compensate for bullet drop at longer ranges, using the "4" for 400 meters, the "5" for 500 meters, and the "6/3" setting for 600 meters.
The advantage of the 25/300 meter zero is it allows one setting to be used to score combat effective hits on a target out to around 350 meters. The disadvantage is the bullet will hit more than 8" above your point blank POA at maximum ordinate, the point at which the projectile trajectory has risen to its highest point above the sight axis, in this case around 185 meters.
It would be nice if the A2 rear sight on the detachable carry handle had a "2" setting to allow you to use it to sight in at 50 meters for a 50/200 battle sight zero, but it does not. Some people modify the A2 rear sight elevation drum to allow it to be rotated to a height below the 6/3 setting for just this purpose.
This webpage has much more information on zeroing the A2 sight on fixed and detachable carry handles than you will ever want to know. It also discussed the USMC method of zeroing the M16/M4 which is completely different from what the Army uses:
http://savannaharsenal.com/2017/04/08/how-to-zero-ar-15-a2-type-sights/