Hi all - I'm wondering if someone can help explain a calculation to me.

I have a crankset, with manufacturer specs of a 42mm chainline, and calls for a 110mm BB spindle (JIS) for "most applications" (it's a VO Grand Cru single). Meanwhile, I have an IGH, for which I calculate the chainline to be 49.5m (a modern Alfine 8, with cog dishes inwards).

In order to match these, do I simply add (49.5 - 42) = 7.5mm to the spindle length, and buy the closest one? That would be the 118mm BB.

Is the calculation any more complicated than that?

If anyone wants to comment on my number for the Alfine 8, or steer me in the direction of anything other than a sealed Shimano UN55 BB, I'm all ears...

Thank you!!

Spindle length also includes the NDS, so basically, you'd add twice as much.
However, some spindles have an offset where the DS may be longer than the NDS.

Scroll down just a bit to the spindle chart and you can see how Sheldon illustrated the "symmetrical equivalent".
https://www.sheldonbrown.com/bbsize.html

I would take some direct measurements before buying any parts. It's possible that your crank isn't actually at 42mm, and your hub isn't at 49.5mm (looks like the chainline at the hub can be adjusted.)

Square-taper bottom brackets lengths vary on both the left and right sides, so it's not just a matter of getting one that is longer by the difference you've calculated. To gain 7.5mm on the right side from a 110mm BB, you would actually need to move up to a 122mm!

Shimano UN54 Cartridge Bottom Brackets Relative Chainline ? Harris Cyclery Blog

Thanks - maybe that's the way to do it. I'll build up the wheel and stick the cranks on another BB, measure everything, then buy the BB last. Sound like a plan?