Joe & Tom,
I understand the concern, but provided there is a positive length location (registering to surface of Chuck) AND the cutting gear train is kept engaged (so nothing can move independently( = picking up the cut) AND an alignment mark is used to control rotational position, then very accurate and repeatable threading operations can proceed. In most circumstances the alignment mark does not need high precision - an error of 0.5mm in rotation on a 40mm dia 1mm pitch thread corresponds to a error at the cutting tool of about 0.004mm (0.0002inch).
With quality equipment, e.g. my precision Pratt Burned chuck, which probably cost more than the whole mini lathe, this is achievable. But even cheap chucks which may have noticeable runout are usually quite repeatable. Even if the error is 5 times worse it still corresponds to only 0.02mm (0.001 inch). Just needs a bit of care to be clean and attentive to detail.
Otherwise, what should a beginner do - at some point it has to come out of the lathe and be mated to the mount - if it does not fit or is too 'snug' - throw it away and start again? or put it back in and take another little cut?
Tom, ideally you should learn to measure and specify threads properly, micrometer and three wires etc, but I have had commercial CNC cut products (thinking Celestron) which do not fit. Maybe before your first real part, you should make a 1/2" thread to fit a 1/2" nut as an exercise; get to know your machine.
I have done what I described above and it works.
PS Its never luck, care and attention to detail wins.