Dunno if this helps, but here is a '70s vintage chart of Stronglight spindle lengths:


It gives two choices for double, and two for triple, with letters after the length — anyone know what those mean?
Double is 118.5 "C" or 120.5 "R"
Triple is 123 "S" or 125.5 "T"

I have generally used the narrow of those two, i.e. 118.5 and 123 respectively.
Those choices are 2 mm (double) and 6.5 mm (triple) longer on the right than the left.

Using triple as my example, to use a symmetric 123, you'd need 3.25 mm of spacer on the right (fixed) cup. But that assumes your spindle has a taper that matches Stronglight's exactly, which is unlikely. So your crank may pull on further or less far, depending on your actual spindle taper, so some trial and error is to be expected.

Some people (probably a lot of folks here) memorize what happens when, say, you put a Campy crank on a Stronglight spindle or vice-versa, but I have never even tried to keep that knowledge onboard, I just try it and see. Given that actual parts you have in hand seldom match the catalog spec exactly anyway, trial fitting is the rule. Often enough, it needs a little adjustment even when you're not mixing brands/models/eras. Yes that's frustrating when you have to buy a spindle to try it, which is why I have such a heavy bin of vintage spindles. That gives me the luxury of never having to use a modern symmetric spindle in a vintage bike. The correct asymmetric spindles come available on ebay practically all the time, so that's pretty do-able if you want.

Of course if you want a more modern spindle or cartridge BB in your bike then I'm not gonna fight you over it, but you'll lose points at a concours!