OK I sighted the thing in, but at 25 yards, it hits a smidge above bullseye but the rear sight is TWENTYONE CLICKS up!

Someone told me that it should be in the ballpark of 8-12 clicks.

I got my windage set to mechanical (?) zero, or whatever it is that the windage is right even with the rear sight centered.

Is 21 clicks ok or am I missing something? I want to get a battle zero.

Edited: my front sight wobbles (rotates around the barrel) a little because I think the muzzle brake is loose.  That muzzle brake is sweet, it makes the thing feel like a 20ga shotgun and blows those empty boxes off the table when I shoot!!!

What do I need to clamp it down good?

First, I'd say move your head forward on teh stock. And get a consistent cheek weld, with your eye the same distance from the rear sight every time.

I'm learning that a CONSISTENT SIGHT PICTURE is VERY important for good groups.

Raising the rear sight raises yer groups. Drop it down to 8 - 12 clicks, and you should be good.

Alternatively, Sounds like yer front sight post is a smidge short.

off topic....

Did you mount the stud for your Harris, yourself?

If so, any pointers?

Rule of thumb -

Rear sight - groups move the same direction you move your sight.

Front sight - groups move the OPPOSITE direction you move your sight.

From the picture that rear sight IS way high. I finally put mine on paper the other day, and 12 clicks had it around an inch and a half or so high at 25, good for the battlesight zero. Make sure you're keeping your head down on that stock, maybe try it without the bipod and use some sandbags.
The front sight assembly (Flash supp. or muzzlebrake uses a set of pliers made for that chore for the castle nut. If it is a new rifle I would send it back to Springfield for fixin'  Hope this helps.

Just did some playin' with/examining/thinking om mine, after looking my sight aperture at twelve clicks is approx. 1/8 inch over the top of the ears.
Something I haven't seen covered is, NM sights vs. standard. NM have 1/2 MOA adjustments while standars are 1 MOA, would this make normal for NM sights 16 to 24 clicks? My M1A has sights that are NM size front and rear, but I don't know about the internals of the rear, and I'm not takin it apart to find out! And make sure to keep an eye on the elevation knob screw as they like to come loose and then your clicks won't mean squat.

On a NM rear sight the 1/2" elevation is achieved by rotating the apeture hood, so for clicks from bottom are the same for NM-standard sights 1 min unless you use one of those glue on disks with 1/2 detenets under elevation knob. Standard windage is 1 minute clicks, NM is 1/2 and can be done a couple of ways. Most common is to use a sight base and windage knob with finer threads another is to rework the windage knob with 8 detents and drill a hole for a spring and ball in the recvr " this is typical of the loaded Springfield rifles "
As for number of clicks up there are quite a few factors involved their ! what sight picture your using etc...