Engine anti ice ON whenever the AFM requires; normally +10C in visible moisture etc, and/or before / when descending. And don’t forget to select it early, well before entering potential icing conditions allowing the engine components to warm up; airmanship / common sense.
BAe146 / Avro RJ AFM changed to include temps below -40 as the result of flight trials. The industry standard for ignoring temps below -40 was based on some fairly old research indicating that icing would not be encountered at temps below -40C as the water content was low and any moisture would be all ice crystals. However due to 146 engine problems where ice crystals ‘stuck’ / froze within the engine, and following flight trials, the AFM was revised. N.B. All engines should now be modded thus the specific problem no longer applies. Similarly for the 146 and RJ aircraft, airframe icing in extreme / very rare conditions could cause an elevator oscillation; again all aircraft should now be modded to reduce the effects, but not completely eliminate them; hence the AFM limit to use anti ice at all temps below +10C.
Just remember that we (humans) don’t yet fully understand the weather / atmosphere; who said that hurricanes never form in the South Atlantic – so where did “Catarina” come from - it hit Brazil earlier this year. And then there wasn’t / was the hurricane in the UK…..