I am going from the suggested 6-8 encounter adventuring day guidelines, in which case 6 difficult or deadly encounters would give more XP than the guidelines suggest.
This misses the point. The guidelines are not the system. The guidelines are not rules. They're advice. They are
only advice. You are not a new DM, you do not need to follow the advice.
Seriously, my point is not a point that can be countered by "but the guidelines", because my point includes ignoring the guidelines.
Stop building adventures using the guidelines. Instead build a world where guards are lethal, where there is a chance of a wyvern hunting the woods near the adventure site way before the PCs can handle a wyvern, and use the lingering injuries and slower healing from the DMG. Next, let the players know what kind of game they're in for, and play. Last, enjoy how the gameplay changes to something more old school, because the idea of getting in literally any fight is scary.
Out of curiosity, which is the "best edition of dnd at making it easy for the DM to accurately set the difficulty."?
4e. Barring the dice just absolutely going weird (every enemy critting while PCs roll garbage, round after round), a 4e encounter is as deadly as the DM makes it. The system for building NPCs and encounters is both robust and transparent, the levels are accurate, and the toolset is incredible in both depth and efficacy. And that's true for combat encounters, traps, hazards, and overland travel.
5e is better in some ways, with more optional rules to change how easily PCs recover, whether lingering injuries are a possibility, etc, but 4e still wins out simply for the fact that the DM can accurately predict what a given encounter will do to the party, while the PCs have no idea before an encounter begins how rough it will be.