A note on older cup and cone bottom brackets. Many were not symmetrical; having the left crank sit closer in than the right. And, at least on some, that left crank offset changed with spindle length. This means that if you wanted to say push the crank out 3 mm, it wasn't always as simple as just ordering a spindle that was 3mm longer or 2 x 3mm = 6mm longer. You had to actually hold them in your hand and measure them and compare. (Race to end for each side.)

The joke being that BBs, like so much of bicycles over their history, are made to standards. Really good standards. And lots of them.

Edit: And this assumes you are using entire matched BBs. Mix and match and another element comes into play. The thickness and shapes of the races which affect where that spindle sits in relation to the fixed cup and directly affecting where the driveside crank sits.