My observation on G6 calibration now that I’ve had a G6 for 7 months:
If my CGM says 78 but my fingerstick says 122, and I really trust the fingerstick, I can do a calibration.
When I punch in 122 for the calibration, I can see that what the meter does is average 122 with 78 and now it shows me at 100. Which is EXACTLY halfway between 122 and 78. This INVARIABLY is how it works.
It “smoothly back-corrects” some of the past hour of readings to show a smooth ramp to the new calibration (not a stairstep).
If I do a really radical calibration, the math I describe above briefly works, but then the CGM decides that it doesn’t trust anything anymore and goes into a “Recalibrate after xx minutes” mode. This used to happen when I had wonky numbers in the first 12 hours of a sensor and tried to “calibrate them out”. Now I mostly do a presoak but I have learned there’s little to no purpose in trying to calibrate the first 12 hours of a non-presoaked sensor. You just have to ride out those false low readings. You cannot calibrate them out.
By repeating the calibration a couple times over a 10-day sensor, you can see what the calibration is doing is a successive approximation. It is always the arithmetic average of the CGM reading with the fingerstick reading.
I don’t know if it’s adjusting both the intercept and the slope of a calibration curve, or just one or the other, but I imagine that internally it’s adjusting both intercept and slope especially if you do some calibrations while high and others while low.
Mathematically if you only calibrate while at the same number, I don’t know how CGM could figure out that it should tweak the intercept or the slope of its internal curve to do a rise-and-run curve.
The meter doesn’t believe any one calibration as gospel. Your first calibration will take the “4-digit code calibration” and average it with your first calibration. So a single calibration is NOT throwing the 4-digit code calibration out the window. But the net effect of several calibrations means that successive calibrations make the original 4-digit code calibration less important.