I have been reading old posts on this subject. One post seems to indicate that 3/4" .75" dogs work in 20mm (.7874" holes). I am still wondering how well. I invested years ago in: Veritas Surface Clamp, a Veritas Hold Down, Veritas Bench Anchors, Veritas Round Bench Dog, Veritas Wonder Dog, all 3/4". I also have a Festool MFT table and Festool clamps which of course uses 20mm.
To make things worse I drilled holes in my compact Hammer workbench using my Wood Owl marked 3/4" Ultra Smooth drill bit, which as it turns out is actually 19mm (.748"). The holes I drilled are just a little too tight for my Veritas clamps and dogs, especially the ones with the long posts.
I hope to remedy the situation by buying either the Veritas UJK Parf Guide Drilling System or a 20mm Dog Bushing with 25mm bit to drill the hole for the bushing and 20mm drill bit designed to be used with the bushing. I am hoping to drill out the old holes with one of the jigs designed to make "perfect" 20mm holes.
The Veritas 20mm Bushing (& bit for it) look like they would be more accurate at drilling through the 3 1/2 and 2 1/2" sections of my workbench but the Parf Guide looks better at laying out holes. I have plans to build other workbenches too, and would like to standardize on the 20mm holes so I can use Parf & Qwas dogs and my Festool clamps.