This.

If your rear end is aligned correctly the distance from centreline to smallest cog is always a shade under 60mm: by definition the distance from centreline to lock nut is 65mm and the usual distance from lock nut to smallest cog is about 5-6mm*. The spacing between cogs for C10 is 4.12mm, so the largest cog is 9 x 4.12mm (37.1mm) further in, giving a centreline to largest cog distance around 22mm.

Your 42mm chainline will normally line up somewhere between the 5th and 6th cog. If it doesn't, something is amiss. Given that you are trying to make this work in a space that wasn't designed for it, I'd start by checking how springing the rear end (to fit the wider hub) is affecting the alignment.

*If the outside of the lockring is 2-3mm from the lock nut. The distance from the outside of the lockring to the centre of the smallest cog is about 3mm, not coincidentally equal to half the width of the chain.