I thought an expression that at 30 fps adds 2, or 120fps adds 0.5, and so on
If you want to basically add 1 to a variable every tick, but take in to account framerate independence, then that's 60 * dt. It will do exactly what you said: every tick it will add 2 at 30 FPS, 1 at 60 FPS, 0.5 at 120 FPS, etc.