Complicating the choice is that many square taper bottom brackets are not symetrical so the drive-side spindle length change isn't simply 1/2 the difference in overall length. These remain a matter of trial and error unless the crank manufacturer makes a specific recommendation as Shimano did for the 6400 Ultegra double at 115 mm or the 1057 105 triple at 118 mm.

Octalink and ISIS bottom brackets simplfied the problem by offering only a limited number of spindles. I.e. road Octalinks (V-1) double cranks took 109.5 mm bb and triples 118.5 mm and that was all that were available. External bearing bottom brackets are even simpler since the spindle is part of the crank and there is no length choice at all.

Also, while chainline can't be ignored I wonder about agonizing over it to the nearest mm. The chain is almost never in a straight line anyway.