UngeheuerLich
Legend
Why should we default to 3.5?spellcasting was always in clear and strong voice.
And yes, 5E is not precise in this as it is not precise in many things, so if we default to 3.5e Perceptions(Listen) DC for spellcasting would by -10 or 0 if being generous.
maybe 5E24 will be more clear about those specifics.
We could as well default to OD&D.
5e is a different game.