I agree with this. Your life would be easier if you took the bike to your LBS to figure out. But; if you’re a hard-headed DIY’er, such as myself, you might try placing the spindle in the BB shell approximately where it lie if it were mounted; then slide the crankset you want to use onto the spindle. Then run a straight line from the center of the freewheel/cassette to the center chainring. If this line runs parallel with the bike frame, you have a decent chainline. If this line is not straight, you at least have a ballpark idea of whether the spindle should be longer or shorter.

If your upgrading your crankset to a more modern 4-bolt, they use much shorter spindles. I found this out when I built an ’03 Rockhopper frame which spec’ed with 110mm spindle. I had a UN54 113 BB that I used with an older XT 5-bolt crank and the inner ring was so tight that it rubbed against the chainstay.