You shouldn't need to use dt anywhere. A fixed timestep of 1/30 is the timestep used per frame. So if the game is running at 60fps then after 60 frames the physics will be 2 seconds along compared to the actual 1 second.
You can specify a variable timestep but that isn't so good with physics, so a fixed timestep is recommended. The drawback is the physics can run faster/slower depending on the screen refresh rate, aka it's not framerate independent. That said you could detect the fps of the game when it starts and set the timestep based on that.
Do I understand this correctly.
If i set my game/level to start with
On start of layout
Stepping mode - Fixed
Fixed timestep - 1/fps
The physics will run at same speed on screens with different refresh rates?
If I leave it at the standard 1/30 does that mean someone with higher fps can finish the level faster?