Bubble levels and digital levels have very clear indications of zero, so I see no concern in determining when it says zero. There can be a concern whether the level is contacting the steer tube clearly, but that is largely a matter of whether the contacting surfaces are clean, straight and free of debris. These two types of levels are much easier to read clearly than, say carpenters' or roofers' angle finders. I use scales and a caliper but in my jobs and education over the years I have been trained in use of those tools. I'm not overly worried about errors. If you've been trained in school in how to use a slide rule (late '60s high school, for me!), you are halfway there.

Plus, for each measurement I wait a little while for the measurement to stabilize. My digital levels can take some time to settle. There's clearly an integrator in the algorithm.

You speak so much of untested, error-prone methods, and I wasn't sure which methods you saw as such. Because you spoke of this in the post following my process description, I assumed you were responding to what I said. I see you were mainly making general comments.

A method that gives you a quarter millimeter is solid if it is consistent physically and in the reading of the result. Part of this is the user and the technique. We're not in the business of providing lessons in how to measure (at least I'm not), so I don't think the skill points in what I'm doing is a problem for me. I have something that works for me with the tools and location that I have. If someone else can get results he/she is happy with in another way, that's fine for me, and kudos to them!