yes, that is correct. But it can be off by a mm or 2 based on how it was manufactured.

If you can go back in time and see how close the chainrings were to the frame before you took it off, it would tell you if you can safely go one size down.

For safety, I usually take the closest size that is exactly the same or slightly bigger than the measurement of the old spindle. This promises you that your chainrings will not grind the chainstay.

The one thing about switching to cartridge that I have never been able to reconcile exactly is that many loose ball spindles are not actually symmetrical. So A and C are not always identical. So, while you should pick a spindle based on chainline, you may end up with your left crank arm being in a slightly different place than before. Not a big deal unless you are really sensitive to your Q factor I guess though.