5/32 or 3/16. You have plenty of exposed base after you traverse the ramp to either extreme. You need a drill bit of the correct size to match the desired size tap. You could use a larger screw, just chose one that bottoms-out below the top of the base when installed. This fix will allow you to adjust or remove the sight down-the-road if necessary.
Thank you for those recommendations! I'm very glad I got a few different fixes to my rear sight problem (and one that I hadn't seen previously at that!).
Zeke H.
"Gentlemen, this is a story that you will tell your grandchildren, and mightily bored they'll be!" - Sir Brian Horrocks, 1944.