For my example the screen x is left/right, y is up/down, and z is front/back.

You need to take into consideration the size of the main part, then calculate half the red Parts X, Y, and Z dimensions. From your picture it looks like it’s 2x2x2 so the values can just be 1. If you have an odd shaped rectangle then it’ll be a bit more difficult.
For the x axis subtract 1 from 1/2 the size of the main Part’s dimension for the red part CFrame X axis.
For the y axis add 1 from 1/2 the height of the background part (assuming that it’s sitting on the baseplate, not inside it),.
For the z axis figure out if the part is being placed on the - or + side of the background part and add or subtract 1 from 1/2 the front/back dimension of the background part.
Cframe the red part at your calculated X,Y,Z Position.