I am not sure your logic is completely correct for comparing the calibration of the Dexcom with the calibration of another instrument.
That is like comparing apples to oranges.
You are taking measurements from any Blood Sugar meter and then inputing that data into the Dexcom. The Dexcom doesn't measure blood sugar. The dexcom only measures the change in the interstial fluid. It does not measure the blood sugar.
So if you insert a blood sugar reading, then it calibrates to that reading you insert. It has nothing to do with the actual blood sugar. You could for example, enter in a blood sugar always 50 points higher than any reading on a blood sugar meter, and then your Dexcom will constantly read a number 50 points higher.
The Blood Sugar meter you purchase in the store, to be accurate should be sent back to Yellow Springs every 12 months to make sure it is calibrated, and when it is suppose to be measuring 1volt, it is getting 1 Volt, +/- 0.2Volts, remember the accuracy from the ADA for Blood Sugar meters is +/- 20%. So every Blood Sugar meter in true honesty is just a guess. Yes, they are probably in the accuracy of 5-8% most of the time. But if your battery is on the edge, and the humidity is on the edge, and the temperature of the room is at the edge and all of the other factors are at the edge, you blood sugar meter,no matter which one you use, is going to be giving you a bad reading.
That is why the Dexcom is best at seeing trends not actual blood sugars