Attaching two models by predetermined points?

My current goal is that I'm building a tool which replicates a small amount of building functionality from within ROBLOX itself, but with preset models which I am also developing for this project.

My problem is: I have multiple models, and each model has within it a PrimaryPart which is its attachment point, which should be placed INSIDE another blocks attachment point, which would cause the blocks to line up perfectly.

I understand that CFrames would be the solution to my problem, but my problem is the math to get to the result which I desire.