1/2' worked for me, at least with a DPP Titanium plate and a 507C (RMR footprint) and my 2.0 standard size slide, which is about .010" thicker than the stock plastic plate. DPP is a very nice plate BTW, better looking and fit, imho, than the plastic plate. It has rounded corners etc.

Maybe this one might work for you? Kinda pricey but I have it in hand and really like the fit. Note, slide it in from the side instead of trying to drop it down from the top. It is a tight fit! Google search it.

DPP Titanium Optic Adapter/Mount Plate Compatible with MP Shield Plus Kimber R7 Mako Holosun 407K/507K

Being a mechanical engineer, I'm still trying to get over mis-matched screw standards. Specifically, the infamous issue regarding the flat head angle. I believe it is 90 degrees on the Holosun (meant to be used with metric screws) vs. the ISO standard (82 or 100 degree) for 6-32 screws.