OK, let me back up a bit!
My mold is for the fuselage of my 428 plane. I designed the plug for the molds all around minimums. as the rulebook state 2.25" square for the firewall, so this is what I used for the original plugs. Since we all know revisions are inevitable, I made some plug revisions and am doing my last checks prior to committing the revised plug to the molding process and don't want to make a costly mistake. Since all I currently have on hand is a Nelson SS for 428, that is all I can measure. To my amazement, the mount was larger than the required measurement! So, in an attempt to have it fit as well as I can, I would like a few back plate measurements. If I make it too big or too small, it is just unneeded drag.
Thanks again,