Yes, I had a supermatic that the 1/2 or 1/10 was off on after getting the rest of the speeds from 1/100 and slower in. I had slightly over filed the position so I used a Weller 8200N and put a little silver solder on the segment and redressed with a file. More than a few thousands may flake off. Adding brass with a brazing rod and torch might work also if one works quickly. The major problem is the dial distorting from the high heat of a soldering gun/iron or torch.

As for speeds above 1/100~1/125 they are controlled by the booster spring(s). I have tried reforming the ends of the booster spring so that it would compress more with no significant effect on the speeds. I have yet to make a new spring to see if that gets the speed up to par.

As for the speed tester I use which is similar to yours I have found it inaccurate above 1/250 second shutter speed. I have taken some known good shutters and made equivalent exposures at several speeds in addition to the fast speeds in question with an 8x10 gray card evenly lit from about 10-15 feet in the center of the frame on ISO 400 B&W film. The gray card had the same density/tone throughout set of exposures, all film processed in the same daylight tank at the same time, scanned at the same manual settings and if read with a densitometer would be within a .1 unit of one another. The tester showed the speeds to be 1/2 stop slow at 1/250 to 1 stop slow at 1/1000. I have tested other shutters that tested 1 stop slow at 1/200 and 2 stops slow at 1/400 to have exposures on ISO 400 film to be significantly over exposed at those speeds.