Aimpoint S1 on a Mossberg SA-28 anybody done it?

[Mullet Hunter]

Trying to figure out the best shims to use to center the site and make it fit right. After measuring the rib the card tells me to use the A &1 shims but that moves the site center to the left side of the rib when I tighten it down, anyone else done this to a SA 20 or SA 28? If so what shim combination are you using? Scratching my head....