First, I accept that nothing is set in concrete with BG.
I have a rigid diet.
My libre scan values have pretty standard variance to BG values when fasting and between meals. Unfortunately tthere are operational difference between sensors and where they are stuck but, once settled I know, for example, that a sensor is apt to be 1mmol low and 20 minutes behind a BG.
If I eat Quinoa (my staple diet) then my BGs increase faster than scan values, peak before the scan and drop before the scan. So, BGs appear to streak away from the scan, peak way above a scan value and then drop below the scan values while the scan is still catching up. It takes around 3 hours for the two values to realign to their constant differences. It's the time lag effect plus operational difference.
I am pretty sure I could predict my BG levels from a scan if I knew the exact pattern my glucose was following. That is impossible so for 3 hours the Libre scan does not give usable data as to what my BG is. That's 9 hours a day. It does, however, reliably tell my where my BGs have been.
Because I have a rigid diet I'm beginning to get a feel for what the Libre is telling me most ot the time but only because I know what has gone before. A Libre scan in isolation is meaningless whereas a finger prick tells me where its at.
This is how the thing works for me. Others will vary I'm sure, especially if they have a life.