I started this thread about a month ago:

https://www.bikeforums.net/showthread...-BB?highlight=

I have a 68mm English BB on an early 90's MTB that will become a SS commuter. I have settled on square taper, but as far as spindle length and adding spacers for chain alignment, I am so lost! Can anyone who has done such a build offer some insight? I want to get the parts ordered this week.

One way to go about it...

1) Determine chainline of your rear cog is. One way to measure: 0.5*(frame spacing) - (center of cog teeth to inside of right dropout)

2) Look up the chainline of your crankset. If it's a double crank, then the inner position will be 2.5 mm less--and the outer position will be 2.5 mm more--than the specification. If it's a triple, then the middle position will be spec, and the inner and outer positions will be 5 mm less or more than spec, respectively. Refer to: https://sheldonbrown.com/chainline.html

3) Above chainline assumes the correct bottom bracket length is used (crank should specify which bottom bracket to use). If you need to adjust the front chainline, you can get a different length bottom bracket. As you increase (or decrease) the length of the BB spindle, the chainline will move out (or move in) by half of the difference (because length is added or subtracted from both sides of the spindle, yet only the right-hand side affects chainline).

Let us know which parts you're working with.

JT offers good advice, but if you want any concrete advice, rather than guidance, you'll want to give us the players here: The rear hub, the cog (or freewheel), and the crankset. Oh, and the frame. and, if your frame has vertical dropouts, your plan for maintaining tension on the chain.