You need to separate chainline from spindle length because they don't equate. Chainline is the distance of the chain's central line from the frame. Ideally you want the mid point of the chainrings to line up with the mid point of the cassette, so the chain is running as close to staight as possible in the most common combinations.

You can get chainline guidance from a chart or tech site, or can measure from the inside of the dropout to the center of the cassette and subtracting from the OLD hub width. That's the target to shoot for, but you may want to tweak it slightly. For instance I set my chainline up outboard a bit, since I spend most of the time on the larger ring and outer third of my cassette.

The reason that spindle length and chainline don't compute is that cranks have offset between the chainring plane and the "hub" of the crank arm. Modern spindles tend to be shorter because modern cranks have the hubs sitting more inboard compared to the rings, mainly to improve spindle rigidity. So a modern 102mm spindle and crank may yield the same chainline as an older 117mm spindle and crank.

If you can source a spec guide for your specific crank, it'll usually suggest a spindle length which will yield a textbook chainline (for that crank).