I've found that my old eyes can focus better on the corner of a square aiming point w/common cross hairs than on the more common circle. I guess I bought the last of the Targ Dot squares a few years back, as I almost never found any back then and do not find them listed today.
The large ones are 2" and the small ones are 1". Choose size based on power of scope and bracket lower right hand corner w/cross hairs. You can actually shoot more than one group on larger targets if you wish for comparison by choosing opposite corners as POA.
I put them on regular 8 1/2 x 11 copy paper and when developing a load, punch target and keep in 3 ring binder w/data. I always make a copy of my zero target and cut it small enough to fit in the lid of my ammo box for that particular rifle/load, making life simpler when picking up a box of ammo for specific hunt/rifle.
If and when I run out I have a backup plan to use plain black pasters on white background (they offer the pasters in various colors) in the pattern shown below. W/standard pasters and 1/4" spacing, the resulting square is 1 3/4". You can vary the spacing to accommodate the thickness of your cross hairs for a very precise hold. Haven't actually tried this one yet, but thinking it might even be better than the Targ Dots.
When forced to shoot on a round target, someone suggested bracketing the circle with your crosshair just as you would a square (vertical line on side of circle and horizontal on bottom). I found this to work better than center hold, and use the smallest dot I can see on these.
Copy paper punches clean with good backer. I mount my targets on old political signs, free for the asking after elections. Much better than cardboard and last a long time.