That's what I tried last time, that's how I arrived at roughly 7 or 8 metres (But I used another calculator http://tawbaware.com/maxlyons/calc.htm )What you need to do is calculate the 'Depth of Field'. With most legacy lenses, it's easy to do because of the engraved aperture/depth of field guide on the lens barrel. Depth of field is a combination of focal length, aperture and subject distance. Here's a calculator that can assist: http://www.dofmaster.com/dofjs.html.
Using this one, it pretty much says my infinity distance is 4.5 metres. Oddly enough I just tested that and it does seem to work, I focused across the room at roughly 5 or 6 metres away, then looked at the top of a few buildings 200 to 300 metres away in zoom mode and they really were in focus.
4.5 metres is a very reasonable distance for me, It would mean that if I had a focus lock feature (hrmmm maybe it does I'll have to look into it now), I could lock my focus at infinity and I'd never see focus speed problems again right? It would also seem to imply, if I took movies, just stand more than 4.5 metres away all the time, and there won't be this focus-hunting problems everyone talks about.
I guess my question would then be... wouldn't this mitigate half the worlds focus speed concerns?
Am I the only one who thinks in such a crazy way? I've never seen anyone ask anything like this before.