On Repitier/Marlin you can set each towers diagonal rod length individually. So you can print a delta tower calibration measure each tower's distance in the X-Y plane use that to calculate a dimensional error to the tower and calculate the corrections for each individual arm. I'm starting to realize if google doesn't find it then it probably isn't an option.
) for a print that lets you calibrate distance to each tower angle.
Its probably not nearly as important as having the average arm height correct and your tower angle adjustments which the Reprap firmware does a good job getting close on the auto bed probe. I moved on to torturing my extruder (you already know) after I knew i could get good dimensional tolerance.
This is separate from the tower height adjustments in M666.
Now the M500 scares me because the calibration also includes the change to tower angles where I was used to keeping those static based on my calibration print. You are probably right I am just inexperienced with such a smart delta controller. It looks like they are pretty stable each time I do an auto level.
Also the M665 and M666 was the initial setup in the guide. My dyslexia almost cause a head crash. So i made the config match the output of M665.
I'm used to a stone axe and just bought a light saber so i need to relearn my swing...
Thanks for your response,