Just a reality check before I go to the range to make sure I haven't missed anything.

Rifle: 20" DPMS RFLR-C with iron sights consisting of a Standard DPMS carry handle and the stock front sight with a Tech sights Tall NM post

Ammo
Hornady 155 Amax
Ballistic Coefficient.435
Velocity (ft/s)2800
Weight (grains)155
Sight Height (inches)2.5
Zero Range (yds)50

Hornady's calculator gives me this for a 50y zero:
Ballistics Results - 308 amax
RANGE (YARDS)TRAJECTORY (IN)
Muzzle-2.5
RANGE= 50  Traj= 0
RANGE= 100Traj= 1.3
RANGE= 150Traj= 1.3
RANGE= 200Traj= -0.1
RANGE= 250Traj= -3.1
RANGE= 300Traj= -7.6
RANGE= 350Traj= -14
RANGE= 400Traj= -22.3


Hornady's calculator gives me this for a 25y zero:
Ballistics Results - 308 amax
RANGE (YARDS)TRAJECTORY (IN)
Muzzle-2.5
RANGE= 50  Traj= 2.2
RANGE= 100Traj= 5.6
RANGE= 150Traj= 7.8
RANGE= 200Traj= 8.5
RANGE= 250Traj= 7.7
RANGE= 300Traj= 5.3
RANGE= 350Traj= 1.1
RANGE= 400Traj= -5

This seems to argue toward a 25y zero, since the trajectory numbers seem to allow for me to hold
6 O'Clock on MGM flashers all the way out to farther than I can reasonably expect to see them without
having to jack with the rear sight elevation.  

The catch to all of my grand plans is that since this is a new gun, and a new front sight.  I have never
shot this ammo or choreographed it in this gun.  I only have 100 yards and one trip to the range to
work with to chrono,  zero and prepare.   Obviously, if I come up with any insurmountable problems,
I will have to transition to plan B, being in this case, a gun I don't want to shoot as much as I want to shoot
this one.

So, other than my plan to shoot a brand new gun in a match, does anyone see any glaring flaws?

Bore sight it.  Too easy.  Set up at least a 10yd offset target.  Make your hi-viz dot or cross for the bore.  Draw a scale above this starting at 1", with hash increments.

For 10yds, the sight should be aligned 90% above the bore from POI at 100yds.

Put the upper in a vice or solid rest, without the BCG inside.  Align it with the bore mark on your bore sight paper at 10yds.  Dial in your sights to 90% height of your sight height above bore and you'll be really close on paper when you go to the range.

Use a level on the upper to keep everything square, and plum the bore sight paper line.

My favorite way to sight in any type of rifle:

Set up a target at 50 yards
Remove bolt/bcg
Look down the barrel
Center the center of the target with the barrel
Do the same with the sights
Reassemble, and take a shot.

You will be on paper ... well, I haven't seen anyone NOT get on paper using this method.

Shoot at 25 yards and see where the group went.   Adjust windage towards center.   Elevation wise, If it shoots near the center, or low, leave it alone.

Shoot it at 50 yards and make sure your windage is still center.  Make sure you are near the center, or slightly low, vertically.  If so, leave it alone.

Shoot it 100.  Make your final windage adjustments and verifications, and adjust and verify for bullseye elevation with your 6 O'Clock hold.

Go back to the 50 and learn where it shoots with your newly acquired 100 yard zero.

Go back to the 25 and learn where it shoots with your newly acquired 100 yard zero.

Learn your "hold overs" or "hold unders" at the 25 and 50 with your 100 yard zero.

Why are you using a tall post?