Here is the definition of bouncing:
Bouncing - Bouncing is simply a natural reaction to what the cat's system perceives as a BG value that is "too low". "Too low" is relative. If a cat is used to BGs in the 200's, 300's, or higher for a long time, then even a BG that drops to 150 can trigger a "bounce". Bouncing can also be triggered if the blood glucose drops too low and/or too fast.The pancreas, then the liver, release glucogon, glycogen and counter-regulatory hormones. The end result is a dumping of "sugar" into the bloodstream to save the cat from going hypoglycemic from a perceived low. The action is often referred to as "liver panic" or "panicky liver". *Usually*, a bounce will clear kitty's system within 3 days (6 cycles).
Sounds like you've nailed part of it
@Ann & Sister. Dallas isn't used to those low blues he saw today. The other factor could be the missed shot yesterday morning. Sometimes you'll get some wonky numbers as the depot rebuilds. Plus 211 and 221 are within meter variance, they are allowed to be out by 20% so those could be practically the same number.
To answer your question about how low to shoot, gradually lower the number as you get data to show you how he does. You've shot 211 full dose, you could probably lower that to 180-190 now when you are around to monitor. Shooting lower usually means flatter cycles. As he showed you on 7/15, a high preshot could mean high and flat or a dive.