Shameless plug (assuming you'll be using an Octalink v1 BB). Shimano made two BB spindle lengths for road use: 109.5mm (double) and 118.5 (triple, with a serious DS offset). Most people don't realize that you could also use their XTR BBs, and those came in 112.5 and 116.
I used the latter two on my single-ring cyclocross bikes, to improve the chainline.
I have a new 112.5 and a used/VG 116. PM me if you're interested.
This is closer to what I'm after.
I want to make sure that if I buy a Hollowtech crank, that I know what that dimension is.
Point #2 here: https://www.parktool.com/...p/chainline-concepts
I don't want to just be randomly buying a used or new crank and slapping a 1x ring on it with an unknown final chainline number.
For the XTR, that may work perfect. I have to space the left crank arm anyway a couple mm for power meter clearance with the brake so the XTR might be great.
With Hollowtech II, you're talking about cranks with integrated BB spindles, so your spindle length is fixed. You can shim the left arm and/or left BB cup to move the left arm out a bit, but then you're decreasing the amount of spindle/crank arm overlap, which can potentially result in damage to the crank if you go more than a few mm.. In order to play around with different BB spindle widths, you'd have to go to a 3 piece crank system (Octalink, PowerSpline, ISIS, square taper, etc) instead of any of the Hollowtech II cranks.
Also, that 43.5mm is in between the 2 chainrings. If you install a single ring on a standard double crank, your chainline will move in or out a few mm, depending on which side of the spider you install the ring, unless you are using a ring that is offset to correct for this...
"I'm thinking of a number between 1 and 10, and I don't know why!"