My starting place is what was in there before, and whether I want the same, or longer or shorter. For a new crank it's a bit harder, but most used to be sold with BBs, or at least suggested spindle lengths in their specs.
In some cases where I had no idea where to start, I used a "best guess" spindle with a bearing and right cup. I'd fir the spindle and use a straight edge across the inner chainring to gauge the offset with respect to the right cup. Once I have that, I can estimate chainstay clearance or chainline, and adjust accordingly.
The above applies to square taper, and/or spline BBs with a spindle and 2 mounted cranks. More modern stuff often has a spindle fixed to the right crank, and chainline is adjusted via spacers.
__________________
FB
Chain-L site
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