What is the Dexcom "20-20" rule?

A tech support guy mentioned it on the phone, but I neglected to ask what it is. Can anyone explain? Thanks.

UPDATE (April 2014): I found this explanation on the Dexcom web site. However, being a millimoler (Canadian, that is), I still don't quite understand it:
[you should calibrate] whenever the sensor is inaccurate. Inaccuracy is defined as when the difference between your sensor glucose reading and blood glucose value is greater than 20% of the blood glucose reading and blood glucose value is greater than 20% of the blood glucose value for sensor readings > 80mg/dL or greater than 20 points for sensor readings < 80 mg/dL.

http://www.dexcom.com/faq/when-and-how-often-should-i-calibrate

It is for calibration, to calibrate ONLY when it's off by more than the 20/20 rule of If the meter shows 80 or less, the G4 should read +/- 20 points. If the meter shows above 80, the G4 should read+/- 20%. Make sense?

Is this explained in the manual?? or is this a solution from users experience ? I have never heard of this rule. Is this also for the older Dexcom's ??

Sorry. I still don't understand.

If your BG meter reads 80 mg/dl (4.4 mmol/l) or lower, you should calibrate, but if the meter reads higher than 80 mg/dl (4.4 mmol/l), the CGM reading should be within 20% of the CGM reading?

Let's say that your BG meter reads 70 (which is 80 or less). As long as your Dexcom G4 reads somewhere between 50 and 90, you're good to go. If it's outside that range, you should calibrate.

If your BG meter reads above 80, let's say 150, then you go by 20% instead. So if your Dexcom reads between 120 and 180 (which is +/- 30 pts, 20% of 150), no need to calibrate. Otherwise, go for it.

Does this make sense?

Yes, that makes sense. Thank you.

Also, no, that makes no sense! (joke). I'm Canadian, so to make sense of the rule I have to convert the numbers to mmol/l, which is super confusing. It's the Dexcom "1.1-20%" rule to us Canucks:

Let's say that your BG meter reads 3.9 (which is 4.4 or less). As long as your Dexcom G4 reads somewhere between 2.8 and 5.0, you're good to go. If it's outside that range, you should calibrate.

If your BG meter reads above 4.4, let's say 8.3, then you go by 20% instead. So if your Dexcom reads between 6.7 and 10.0 (which is +/- 1.7 mmol/l, 20% of 8.3), no need to calibrate.

Joking aside, thanks for explaining it. ( :