Chef, those Osborne tools you provided a link for appear to be the same end use as an adjustable stitch groover, which I do not like. I have a set of dedicated Tandy stitch groovers set at 1/4", 5/16" and a free hand groover. These are just the "cheap ?" version they sell for around $10 now.
The photos below show a couple of things you have ask about lately. The first is the layout for my tooling area on a pouch sheath so the tooling is confined to the front of the sheath only. This was referenced in another thread "How many sheaths"... If you look closely you can see two dots about 1/8" inside the scribed line. This is the true center line of the sheath. The lines were done with the second wing dividers in the photo below.
The second photo is a shot of some of the wing dividers I have. I use the two smallest ones the most. The smallest I have had for about 40 years, and I don't have any idea where it came from. The next one was a gift from a boot maker friend and it has no markings at all, but it is the one I use the most of any. Third is the Osborne 6", and 4 and 5 are India imports and I think I picked them up at a garage sale or two. The second one in the photo which is my favorite and the one used in the DVDs has a max spread of 3 3/16". The first little one is the one I use to mark the stitch border around the window of inlays.
The 6" will do everything the other ones will do but sometimes the smaller size is just more handy.
Paul