Witterings wrote: ... but the inconsistencies trying to use a plumb line off the front of the saddle nose to the bottom bracket .... I can take a measurement one minute and go and double check it a minute later ... the bike's at a slightly different angle and my reading can be 10mm out ..
Did you build my house?
I assumed that some things were self-evident.....
Start with the bike level. On my garage floor, I use a bit of wood of the appropriate thickness so that the tyre contact patches are level....checked with a six foot spirit level....as somebody said, reverse the level to check the level against itself. I imagine you need to account for suspension sag if you have suspension, but that is outside my experience.
If the steering flops about, tie it straight.
Sight down the string of the plumb-line, so you can see if obstructions are biasing it, and sight down the plumb-line to the frame, avoiding parallax errors; if you need to, temporarily attach a scale or something to the frame to sight the plumb line against.
With a laser level can you not lean the bike against a wall (tyre contact patches level, steering straight ahead) put the laser square to the bike and project a plumb-line on either the BB or the saddle and measure setback like that?
If you find all this too tiresome, you can always use Brucey's trick and put a mark on the top tube exactly above the BB spindle....you only need to do it once, you could do it with your laser level, and after that you can work from your mark....but you still need to measure level and square from the mark.
My Web page is about setting up your riding position, not copying your position from one bike to another. To set saddle setback, I mention KOPS as a rule of thumb, and also the no-hands balance test, ie not falling forward if you don't support yourself on the bars. Either method should get you "near enough", with the caveat that they may not be applicable to riding positions that differ greatly from mine.
I suggest there is an "acceptable range" of riding position, and also that riders are more tolerant of variations in (say) reach than saddle height.
However careful I am in copying riding position from one bike to another, when I get out the winter bike I expect there will be a little burst of tinkering with the riding positions because one bike "feels different". This prompts me to ride them all and make comparisons, and often an adjustment here or there. I think its worth measuring carefully to duplicate positions, but the proof is when you ride the thing.
To take your example of a 20mm discrepancy between 2 bikes in saddle setback; that would be immediately obvious to me in a few yards of riding, and I would correct it.