Here are some instructions on how to zero an M84.
Adjusting and Zeroing the M84 Posted By: Pershing Darrow Gervais
Date: Thursday, 16 September 1999
The M84 telescope is equipped with a post and cross-hair reticle that may confuse the shooter previously experienced with cross-hair reticles only. As some have discovered, the heavy post subtends about 3 m.o.a. and obscures a significant portion of the target if it's intersection with the cross-hair is used as the aiming point. Also, the cross-hair appears about 12 m.o.a. lower than mechanical/optical axis of the telescope, possibly causing some rifles to shoot abnormally high at short range. In short, use the top of the post as the aiming point as you do with the iron sights.
This procedure is used to adjust the mechanical limits of the windage and elevation knobs, and to mechanically zero their respective scales to the rifle on which the scope is mounted. The windage and elevation knobs are held in place buy the larger of the two round nuts in the center of each knob. The larger nut secures the knob to the detent, which in turn is secured to the knob shaft. The small inner nut prevents the larger nut from backing off the shaft when making adjustments to the knobs. The round nuts can be turned with a flathead screwdriver with clearance ground out to clear the small inner nut.
First hold the knob in place and unscrew the larger nut about 2.5 turns or until it is stopped buy the small inner nut. You can now disengage the knob from the detent/shaft and turn the knob independently of the shaft. The detent/shaft is capable of approximately 720* of rotation while the knob is limited to about 350* when secured to the detent.
If you could not previously zero the rifle because one or both knobs had reached their mechanical limit, you can now adjust these limits. When the knob is at it's limit, lift the knob to disengage the detent/shaft and rotate the knob X* opposite the direction in which you reached the limit. Lower the knob to engage the detent/shaft at this new point. You have now moved the mechanical limit X* past the point of the previous limit thereby allowing further adjustment in that direction. You should again try to zero the rifle and repeat this adjustment as required, remembering the rotation of the shaft/detent is limited to 720*. After the rifle is zeroed, mechanically zero the scales on the knobs as explained below.
If the rifle was previously zeroed, and you haven't moved the knobs, you only need to lift the knob to disengage the detent/shaft; rotate the knob independently of the detent/shaft until the correct point on the scale is aligned with the pointer; lower the knob to engage the detent/shaft and tighten the larger nut. Do the same to the other knob and your done and you are finished. "