Been looking for a lathe in decent condition for a good price for years now, but never pulled the trigger. In the middle of another project (will post once finished, or at least once I have most of the parts made) I was thinking about possibly needing one to tune a fit. Didn't find anything that matched my criteria (which, granted were/are quite unrealistic, was hoping to get lucky) when I realized that I already have a 'lathe', just no chuck for it. Fast forward a week or so, and I bought a cheap ebay-chuck, and made a backplate to go onto an iso40/sk40 shell mill holder, to go into the horizontal spindle of the mill.
I measured the runout right behind the jaws, and it came out to around 0,05mm (that'd be ~2 thou) which I'm completely fine with, considering the quality of the chuck, and it being an independent 4 jaw, I can tune in the parts, should they need it.
Edit: I tuned it in a bit since then, and managed to get the runout down to around 0,036-0,038mm (around ~1,5 thou). At one point I had it down to 0,024, but that was at the start of a 2-3-hour session of loosening the screws, moving stuff around, clocking the whole chuck, and at that point I figured I could get someething better still. Fast forward a few hours, and I was more than happy with the end result, even with never having exceeded my I think second or third try, after being beaten down by the passage of time. That, and standing in the same place in front of the mill, leaning over slightly, and not knowing enough to interpret the inconsistent readings.