There's a way to eliminate the uncertainty. You need to have a crankset before you choose a BB. (You can accept the manufacturer's recommended BB length for the crankset, or you can measure the crankset and be sure.)

Your chain line is indeed 42 mm, dictated by your Nexus IGH. It's the distance from the middle of your OLD to the cog.

On your crankset, measure the distance from the chain ring to the bottom of the recess where the crank bolt goes. (The advertised BB length is the distance between the ends of the tapered portions. Those points are roughly where the crank bolts will bottom out.)

1. Draw a simple diagram of your BB shell and crank as viewed from above.

2. Show the distance from the middle of the shell to the chain ring as 42mm.

3. Add the short distance from the chain ring to the bottom of the bolt hole. You now have half the length of the BB spindle. Double that number, and that's the length BB you need to buy.