WELLUE 02 Ring - off readings with different finger

[MCS2014]

I always use my index finger on my left hand (I'm right-handed) for my readings. Because I had to use the Virtuox per docs order on my index finger, I placed the 02 ring on my middle finger. The readings I'm getting are way off my norm of a minute of less <90% Sp02, like 26 minutes! This has now happened on both nights on the middle finger. My mask seal shows 20/20, so no apparent leaks.

I just did a search for this and the manual says to not use middle finger. Any reason why?
https://manuals.plus/viatom-tech/wellue-...connection

[S. Manz]

I also have this Chinese O2 Ring and I wear it on my left thumb because the thumb seems to be the thickets of the five and thus provides a tight contact surface for the ring's LED light .
The stronger your Pulse Index (the little bar that goes up an down) the better the reading and yes, I find my SpO2 average to be lower by about 2% on average as compared to wearing my finger tip oximeter.