Unity for any gain or volume slider or knob means 0 - that is when said fader or knob does not alter the volume in any way.
You are asked to have peaks of -1, yet you set it to -1.5? And isn't it the result that matters, not the "setting"? So if -1.5 setting yields -0.1, then -2.4 setting should yield -1, and -2.9 should yield -1.5.
dBFS stands for decibel (at) Full Scale), which is technically the correct way to refer to level settings in plugins and mixers. 0 is full scale > no alterartion, and minus settings do indeed subtract exactly that from the actual level of the audio file.
It may seem odd that "full volume" is at zero, but it actually makes sense, as there is in fact no sound in your DAW or mixer - just "signal" (that has yet to be converted to actual sound by digital to analog coverter>amplifier>speaker)
But I do not know why setting it at -1.5 doesn't yield that - or is there region gain applied on that clip? I can't see that in your screenshot.