Re: Drastic BC change from 800 t0 900 yds??
How much are you off (in a real unit other than 'clicks')? ;)
Assuming this is consistent and repeatable across multiple sessions, one rational explanation is that your scope's tracking could be off in that range of adjustment --
might want to verify your scope's click values in and around the "click range" where you're seeing the weirdness...
One experiment you could do: If you have a reticle that you can use for hold-offs, you could dial up into the range where everything is "working", then use the reticle to hold off for the rest of your elevation --
So just for example (example values here) -- if your verified 800 yard dope is 18 MOA and your 900 yard calculation says 22.25 MOA -- you could dial 18.25 MOA and then use your reticle to hold "up" another 4 MOA. If that is "on", but when you dial up to 22.25 MOA you are off, you know something is up with the scope.
Make sure you're at the power your reticle is calibrated at if the reticle is SFP (and preferably, you'd want to confirm your reticle's calibration).