Looks like you could start with a short flat and that will have some adjustability with the threaded shaft the trigger shoe mounts to. It is a pretty nice system to custom fit it to your hand. I started with the short flat shoe and ended up with the medium flat.
I have a 9mm and a .40 and I did have to use a dremel sanding drum on backside of the grips on the 9mm to make weight. The bigger bore of the .40 barrel took enough steel off to make weight with that gun. Disappointing to have to modify the grips, but it only took 15 minutes and I was good to go.
I’ve been very happy with the Boss mount and RHT holsters. I have 3 belts set up with them and the Boss is a solid piece. The RHT’s fit is different, bit “loose.” But,I find them rigid and fast on the draw with no binding.
Mine came in yesterday and I loaded a quick 500 rounds after setting it up last night. The Sharpie tips needed to flattened a bit to leave a wider stipe. I also slipped some washers on the Sharpies to add some weight and put a bit of extra pressure on them to make the lines a bit more pronounced. Very happy so far!
My .40 Hundo that came last week had a few that were tight also. I ran a nylon bore brush through all of them and the rounds drop in and out freely now. Probably some residual gunk from manufacturing the gauge. Easy fix for my issue, hope yours is as easy.