Ah yes, I think would be better they bury that option under "Advanced use"
The default should be fine - you could check that against a steady (form memory) 60 MPH - if there is something wrong with your car then a difference in numbers will be apparent - but I would expect that to also show up as a per-journey-plan difference too, so ABRP default and check the overall journey comparison
If you got a close-figure for that "model journey" you did I reckon all is fine (and that is your car's range - plus using ABRP for other journey planning / experimenting should be accurate enough). If the figures are different you could do the steady-speed test, and then put that into ABRP config and see what the comparison for that journey is (but I'd be questioning why your wH/mile base figure was different to ABRP)
I'd expect more range on the M3P - the SR+ is lighter, so better wH/mile ... but larger battery on M3P wins