You only experience "parallax error" if you hold your head in an inconsistent position from shot to shot.
The AMOUNT of maximum-possible-parallax error is easily calculated.
In the maximum-error scenario, you will be holding your head so offset from centerline that you will be viewing the aiming point of the reticle at the farthest edge of the visible field of view. And who does that anyway?
The point being, you can calculate the "error," but in practice, it is very unlikely that you will ever EXPERIENCE shot-displacent in a magnitude anywhere near the amount of of calculated maximum error.
If you are considering putting a 3x9 scope on a .22, I'm guessing you aren't needing it to shoot to Olympic-class accuracy standards anyway. And further, the poor optical resolution, and the reticle TRACKING ERROR of a cheap scope, and the questionable benchrest skills of most shooters, and in addition the poor installation of the scope, are usually the real reason people report dissatisfaction of the performance of a scope. But feel free to blame it on "parallax." Most people refuse to understand what parallax really is in the applied setting. So blame it on "parallax."
But, if you want to calculate the MAXIMUM POSSIBLE parallax error, there's an easy-to-use online calculator for that. You will need to know:
- size of front objective lens
- amount of magnification
- factory-determined parallax setting (most centerfire "hunting" scopes are set for 150 yards, rimfire usually 50 yards)
- firing distance
Here's the link:
https://www.lelandwest.com/parallax-error-calculator.cfm
And feel free to run the numbers yourself:
Formulae:
- Parallax error in millimeters (Pmm): ( 0.5 x D x ( Abs ( R - P ) ) / P
where
- D = Objective diameter in millimeters
- R = range to target
- P = fixed parallax setting of scope
- Parallax error in inches (Pin): Pmm x 0.0393701
- Parallax error in Minutes of Angle: Pin / 1.047 / ( R / 100 )
- Parallax error in Milliradians: Pin / 3.6 / ( R / 100 )
Your best bet FOR ACCURACY on a .22 scope is the most expensive FIXED POWER scope you can afford, and the most expensive mount you can afford. Mounting it on a really expensive .22 rifle is usually clutch too.
Otherwise, slap that 3x9 on there, and get the best zero you can off the bench, and don't mess with the reticle adjustment further. Pick one type of HIGH QUALITY ammo too, and stick with that one type of ammo. If your ammo comes in a milk carton, don't expect accuracy.