> what if someone invents a CGM that goes down to 30? 

They will hopefully not be submitting events with `"units": "mmol/L"' and we will take it as is.  They also have the ability of attaching the `noUnitConversion` flag to their event submission.

> A 39 in mmol/L corresponds to 702 in mg/dL. Within the range of possibility.

Not *really* within the realm of possibility.  At least, current hardware doesn't support it.  If some hardware does eventually measure that high, they should be able to do the conversion on their own side.  I forget the exact numbers, but the FDA doesn't really impose any strong accuracy restrictions on finger prick meters for stuff above like 400 or 450, and once your meter says you are at 250, you are just as likely to be 220 in actuality as you are 300.