After a few hundred prints, you know what you;'re looking for on the first layer. I'll watch the brim print, and if something needs a bit of a tweak, I can nudge one or two of the bed level screws on my machine to tighten or loosen the first layer where it needs it.
It's not an unreasonable request. With expansion and contraction of machine parts with temperature, different filament properties, etc ... and for me it's not about knowing that the first layer height is within a couple hundredths of a millimeter, it's about getting a clean first layer that's going to bond well enough to the build plate.
If a machine doesn't have an accessible mechanical adjustment, a little nudge during the first layer print can be helpful.