Hi,
Almost same behavior with my mother having ipf, Spo2 94% to 95% at oxygen 2.5 l/m while she is sitting. I have long oxygen supply tube of 30 feet from concentrator, whenever she walks I increase the oxygen to 4,5 litre which keeps her spo2 level avg 85%. Doc advised that she should walk little and perform easy exercises daily and he advised to increase oxygen supply upto 5 l/m at that time only.
I use portable concentrator simplygo mini for her when I take her outside.
As per Doc and experts nasal cannula work upto 5 litre/m oxygen, above 5 use mask.