ill try it with that, and sorry i cant test that right now
"To elaborate, in the following picture (sorry about the quality…) I want to find the players (P’s) RELATIVE y offset. RELATIVE to the cube’s own coordinate system and rotation. I can’t do it in world space because no matter what orientation I place the box I want to get left and right across it as a y value. "
Inside the Player you get a reference to your cube, i think like you did in the picture before.
Call GetActorLocation from Cube (Vector 1)
Call GetActorLocation of your PlayerActor (Vector 2)
Then substract them. Vector2 - Vector 1 → and you will get a Vector which has the direction Cube to Player
That means break down to the y value of that vector you would have the amount of y units you need to go to get to the player´s y-origin position (the mentioned offset)
Otherwise substracting Vector 1 - Vector2 and break down the result to y component you will get the y value of how many y units you need to go to be at cube´s y-origin position
maybe that helps 