jgsugden
Legend
If you're a DM and have not read the DMG essentially cover to cover, you're less prepared than the designers of the game intend for the DM to be. They didn't write the DMG for it to be ignored - they wrote it for the DM to know and use. It is part of the game RAW.The rule is so surprisingly uncomplicated that it is almost hidden in the DMG. We also know that a lot of people don't actually read the DMG so I think the OP is excused for not noticing it...
I use the rules from the DMG, but the way I set my DCs is a bit more complicated. I figure out who made the secret door, and why, and how skilled they might be in doing it. Then I set DCs according to the capability and resources of the being that made it.
I also have semi-disadvantage and semi-advantage options in my game. If you get semi-advantage, you get to reroll if you fail by 4 or less on your first roll (and you then use the higher roll). Semi-disadvantage forces you to reroll if you get succeed by four or less (and use the lower). For passive perception, it gives you +3/-3. This makes passive perception modifiers go from -5 / -3 / even / +3 / +5. That gives you a big range. I use these disproportionately with perception and investigation, and less often with other d20 rolls.