I am not familiar with that rail, but a 1/2" error is pretty bad mistake to make. Have the gunsmith replace the rail and modify it correctly is the right thing to do.
A home remedy is to shim the front sight by bending a "V" shape shim to put on the side of the rail. I have made a few to fix the same issue by using a piece of air condition duct material cut to size. and then bent in to the "V" shape. A thin bead of silicon can keep it in place until you get the sight installed and tightened down. I use a chamber laser to bore sight, and the goal is to get mechanical zero on the rear sight with laser on the target center. You might have to try different thickness shim material to get close to MZ. Minor adjustments can be done with a thin shim like a soda pop can. This fix works for clamp design sights like KAC, but do not work with Magpul Gen 2 style polymer that slide on the rail (it's to tight to slide the sight and get the shim on the rail too).
Hope this gets you started, PM me to set up a phone call for more detailed instructions.
