Difficulty using CFrames to get the "base" of a wedge

I’m trying to make a wedge to stairs plugin and I’m trying to get my “origin” point which is going to be at the base of the wedge (the blue part is where I want it) I’ve been messing around with cframes and I still cant find the expression that gives me the correct output, does anyone with more cframe experience know how to do this? I have looked on the CFrame articles on the Developer site and also some other guides, but nothing there has helped me.
unknown

Here’s something I did to get the part at the lower base

game.Workspace.Part.CFrame = game.Workspace.Wedge.CFrame * CFrame.new(0,-game.Workspace.Wedge.Size.Y/2,-game.Workspace.Wedge.Size.Z/2)

Follow up of my thread I been having an issue on:

I’m getting the part’s front face. Then doing the same on the Y.