I have an Orbea Gain (which has the same motor and controller as the Ribble) and I've experimented with different power levels, primarily with power assistance level 1. The conclusion was that level 1 at 100% was too high for general use, 60% too low with 75% seemingly the sweet spot (for me). I left assistance levels 2 & 3 as they were as I appreciate the assistance they give on the hills.
Interestingly whatever power level I’ve used it doesn’t seem to affect the range too much, in fact some of the higher power assist levels actually give a better range than some of the lower levels. I haven’t got my head around this yet, my only thought is that the ebikemotion software guys produced an ‘ideal’ zone of about 12 to 14mph where everything just sort of clicks together and runs efficiently