Since I'm no longer using my Technics footers, I measured one and the stud is just short of 1/2" long. Of course, the adjustment range would be less than that as you'd need to leave a few threads screwed in to the base for stability, so Tvad is pretty close, maybe just a hair more than 1/4" total.
The studs on the Parts Express cones I use are much longer, so the range is probably 1/2" or more. Since the cones also have a threaded receptacle, you'd need to leave a few threads screwed in on both ends. If you need an inch or more of adjustment range, I don't think they'll do it unless you buy some threaded rod and make longer studs.
Having an inch of M6 threads exposed would be far less than ideal. I've thought about applying some Loctite to the cone ends, but haven't done it yet. That might lend a bit more stability. You could also try nuts to back up against the base and cones. If my floor were as uneven as yours, I'd make wooden spacers (in place of the coins) that would get you in the ballpark of level, then only do the fine adjustment with the feet.
David
