I guess it's self-explaining :D

Here some math

FirstVolumeSite= Max CD/DVD/BR capacity - Setup.exe size - 14 MB = x
CustomVolumeSize= Max CD/DVD/BR capacity - 14 MB = x
ThresholdPercent= If you set to 100, the complete game will packed in a single archive. Not a good choice if you ask me (especially if the archive get's damaged in some way). Set to 1 and UltraArc will create dozens of archives. Leave it at 30.

It's always good to have some MB left on a disc.

Thanks again for your explanation

Maybe like this?

[Split]
FirstVolumeSize=23000
CustomVolumeSize=23200
ThresholdPercent=30