Thanks - making more sense now. So I am at about hour 15 of a long print and had a failure. I'm not too sure when 😞 I resumed the print and will see what happens ......
In any case - the M591 D0 now reports
Pulse-type filament monitor on pin e0stop, enabled, sensitivity 7.000mm/pulse, allowed movement 75% to 125%, check every 15.0mm, measured sensitivity 7.854mm/pulse, measured minimum 80%, maximum 93% over 369.1mm
Nothing has been changed except this print does have a fair number of retracts. Several questions:
Does the measured sensitivity going down suggest slippage in the sensor (it's two wheels one of which "encoded" with holes) ?
Looks maybe like the minimum % threshold may have been triggered -- hard to tell as I took the above measurement after the restart (was not thinking)?
Beyond checking the mechanical aspects of the sensor - would setting to E21 (i.e. say 3 x nominal sensitivity) tend to make sense for tuning or does that seem a bit course ?