There is no “calibration” what you are seeing is the deviation in your load.
I think you are saying your extreme spread at 1000 yards is .3 tall, yep welcome to long distance shooting, and think about what you asking.
You have no clue what your MV will be until you shoot. You can look at the range, but the reality is, it’s unknown until you shoot.
So how can the Xero know or be calibrated to something external?
Apparently I didn't explain my "without a chrono" process - it involves shooting.
1. shoot and adjust until your have a 100 yard zero - slip or not, up to you.
2. shoot 200, adjust scope until point of aim equals point of impact. Write down your scope setting.
3. shoot 300, do like 200 and write down the scope setting.
4. repeat out as far as you want. You now have a list of elevations for 100, 200, 300, etc.
I use Applied Ballistics.
5. using your ballistic calculator, put in your bullet, scope, and make up a velocity - doesn't matter. Tell it to compute a trajectory where the longest range matches the longest range you shot - if your max range was 800, tell AB to make a trajectory out to 800. If the AB elevation is less than you got, your velocity guess was too high, make it smaller and try again. If the AB elevation was bigger than your measurement then your velocity guess was too low, make it higher and try again.
For example, suppose I shot out to 1000. 300 PRC, 230 grain Berger hybrids. Using my scope, my measurement is 7.6. Just for the helluvit I guess the velocity is 2700 fps. Put in the bullet and my atmosphere and AB says up you need 8.1. 8.1 is more than 7.6 so that velocity estimate is too low (the faster the bullet goes the less drop) so I need to try a higher velocity. In AB, I change my velocity guess from 2700 to 2800 and recompute. Now AB says up 7.4 mils. My AB guess is less that my actual shooting measurement so my velocity guess is too fast, try a lower velocity. Try 2750. Now AB says 7.7. Try velocity 2775. Now AB says 7.6. Since AB is estimating the same as what the gun shoots, I have a working velocity.
Now check the other ranges. The scope elevations should all match. If they don't then there is something wrong with BC or your scope is not working correctly or there is something add about your range - sun, hills and valleys - something.
I have an Oehler chrono but in order to use it, I have to go in front of the line, set up tripods, rig up the screens, run wires etc. If it isn't just right, I have to close the range while I fiddle with it. The above technique gives me a good working number that matches reality. I am asking about the Garmin because the technique above gave me 2725 or a bit more but Garmin gave me 2775 - those are 0.3 mils different on the target. Garmin doesn't know if there is something odd about my gun but the target is always right. I was simply trying to get a read on calibrated Garmin accuracy because if the Garmin is right then either there is something odd about my scope/gun or about the light/wind/hills at my range. It would be nice to know.
